I have bought the Reface CP few days ago. I am connecting it via 12 channel mixer which has no gain knob on the stereo channel, it only has gain on mono channels.

First, you are obviously new to mixers... no mixer has a “gain knob on the stereo channel” — (sorry, that’s a giveaway). No worries...

So before you conclude that something “simple sucks”, let’s learn a bit about mixers, gain, and gain staging.

Gain, on any mixer, is the way you prepare a mixer channel for incoming signal. This is as much to deliver good quality sound, as it is to protect the circuitry of your mixer. The stereo channel typically takes incoming signal from each of the 12 channels of your mixer. It would not have a Gain Control on the Stereo channel, period.

The Gain Control (called Gain/Trim) because it is there to regulate a wide variety of input strengths. Different sources have different output levels. It prevents you from OVERLOADING and damaging the channel. Typically, a mixer will have a switch MIC/LINE.
Mic is for very weak signals and uses the pre-amp to boost signal.
Line is for stronger signals and you may have an overload or peak indicator so you do not damage the circuitry with too much signal. The peak light will flash on as you get within a few dB of the recommended maximum.

Microphone inputs are the weakest of all signals you would ever plug into a mixer. A typical dynamic microphone generates its output voltage by moving a tiny magnet (attached to the diaphragm) in the field created by a coil of cooper wire... this induces a tiny voltage... the Gain on the mixer will take this small signal (which can be as weak as -60 through -50dBm)... and boost it... 0dB being optimum.

A guitar or bass are usually the next weakest signal. Again, you have a magnet (one or more per string) you have movement in a magnetic field inducing voltage... but the electronic in these instruments put out a signal between -40 and -30dBm.

A consumer electronic device like a CD player or small recorder might have an output of about -10dB (we are now getting into the level where it is consider Line level). Items considered Line level will deliver between -10 and 0 and as high as +4 or +6 dBm. Because music is not a steady signal (is not continuous) it is measuring constantly varying signal.

The Gain/Trim Control is there so you can boost weak signals and attenuate (turn down) the stronger signals. This is because each audio device can have a different output level. Sound people exist to do the balancing of these wide ranges of signal strength. It has nothing to do quality. There are extremely expensive microphones costing more than your car that can’t output -50, and there are toys that can output tons of gain. Output has nothing whatever to do quality... many musicians learn this the hard way... they judge speakers by picking the loudest ones (they find later, that they should have turned up the softer one because actually working the volume knob is easy... loud speakers are efficient, that means little if the sound they deliver “sucks”. The best speaker likely eats power for breakfast, feed it to get its best result). Louder is not always better... learn to make the proper adjustments, so you can really judge!

If you post the name of your mixer we can tell you exactly the specifications, or you can look them up yourself. But we are certain a fully functioning reface CP will deliver sufficient signal for your mixer to not “suck”.

If you need help, stop by your local music store, they will be happy to see you and they will go over how to “Gain Stage” your mixer.

Now, if you cannot get enough output from your reface CP, it is not because it doesn’t have enough output. It most certainly does...
It indicates a problem. Here is a checklist

_ Start with the power supply... are you plugged in with the appropriate (provided) power adapter? If using batteries, make sure that lights are not flashing indicating weak or failing batteries.
_ Make sure you are using the proper signal cable (commonly called, a guitar cable). TS (Tip-Sleeve) connector on both ends.
_ Try a different channel on your mixer, remove any Pad (a device used to attenuate strong signals to a mixer channel)
_ make sure you have turned any Phantom Power (+48V) Off on any channel not requiring it. (FYI.. this is only for condenser microphones requiring an electrical charge to do its thing. This can harm items that do not need it.

Let us know.

Extra Credit:
The reface CP is a dynamic instrument, this means it responds to how hard you strike the keys. Unlike the other devices you mention, the CP, like a piano is louder the harder you hit it. This is referred to as “touch sensitivity” (also called “velocity sensitivity). This name of the instruments it emulates are in the piano category (pianoforte)... is the proper name, means literally soft-loud. Please take that into account when judging/comparing to devices less sophisticated.

Thank you for your broad reply. The English language is not my first language, so apologies for using the word "suck" as it is a bit stronger than I understood it.

I am indeed not an expert at all if we speak about mixers. however, the modern versions very often have a gain knob on all channels, including the stereo ones (at least that's how the knob is labeled on eg, Mackie ProFX, Alto Live). Because I am not an expert and the modern synthesizers never required me to use the XLR inputs, thank you very much for helping me to realize that I can use the "mic" input for lower signal instruments. I am using Mackie VLZ4 (I read the manual again now), and I think that Reface CP, as an instrument emulating the old equipment, makes it very accurate. As soon as I input my Reface CP to the "mic" input the signal was significantly stronger and finally I can play a light chord over heavy synth and drum sounds and you can hear it on the "top" of everything, Routing the Reface CP through regular line in input results in the signal being low and I had to strike the keys much harder, loosing the more delicate sounds the Reface CP plays..