I upgraded the BT-01 firmware from 1.04 to 1.07 but nothing changes, it still does not work.

? ? What doesn’t work?
It doesn’t connect? It connects but the App doesn’t function? Please clarify... are you getting an error message?

I cannot check on your version of iPad but perhaps it is a security issue on your iPads.

My MD-BT01 works just fine on iOS 14.4 so that should not be an issue.

You don’t say which BT01 you have... (this would be helpful since there are two products).
Does the LED light up when connect the MIDI Out of whatever you are using to the MIDI In of the BT01?

So unless Apple has broken MIDI again (lol), it might be something else.
Sorry, I don’t know what app you are attempting to use. Do you have another app that can use Bluetooth?
Download any MIDI based Bluetooth app (free) and test it...

Please let us know... What you find.
Extra thought — Bluetooth can take a minute to connect... use extra patience, once connected it remains solid. Take your time.

As I wrote, I use OnSong with MDBT01, I don't have any error message.
On my Ipad IOS 9.3.5 I can send hexadecimals attached to my lyrics that can switch sound on my keyboard.
On my Ipad Pro IOS 14.4., same settings, but I can see the MDBT01 connected but seems that nothing happens, my keyboard does not switch the sounds

https://onsongapp.com/docs/interface/menubar/utilities-menu/editors/midi/settings/sources/bluetooth/
https://youtu.be/RoNUmRPL_iA

I have also read that sometimes you need to find software that does have more direct support for Bluetooth MIDI (the link claims Onsong does not have this) and pair with that other app - which helps all of the other other apps without native BT MIDI support.

You may also find an app that natively supports Bluetooth MIDI and can send/receive to other ports. Onsong has "virtual MIDI" for communicating between apps - so this may be of benefit.

These shouldn't be required. I'm guessing after you pair and the MD-BT01 shows as paired - you should be "good to go".

I guess the other question is - do you have Onsong premium on both your iPad (9) and iPad Pro? Onsong shows the MIDI features working with Premium and not Free or Essentials.

There's a way to configure the MIDI commands with channel ("@" symbol) - may want to be sure all of that configuration is the same. No "@" symbol means to transmit on all channels.

Actually, I'm using OnSong bought on Appstore before premium exists, I bought separately the midi add on.
here is the same hexadecimal sent from both Ipads and from OnSOng to change the rythm on my keyboard :
MIDI: F0 B0 63 02 B0 62 40 B0 06 03 B0 26 5B
Ipad with IOS 9.3.5 works flawessly, Ipad pro with 14.4.1 does not (Same settings on both Ipads)