If the EQ sounds fine on the house speakers - then the appropriate place to adjust EQ (if that's the issue and not level) - would be on the mix being sent out to the IEMs. You wouldn't want to adjust EQ on your keyboard because that would affect everything - not just the IEM. It's possible the signal in the monitor mix cannot be individually EQ'd. You can always also try panning (again on the mixer) to bias the position to one side off-center. Not completely - but 25%/75%. This way, you could use pan to control the relative level of your keyboard vs. the other instruments.

If you want to control your own destiny and not have the sound engineer handle this - then you could throw some hardware at it. A low-cost small-form-factor mixer with very few channels will work (with an AUX send) assuming there's an output-stage EQ. Route the AUX send from this mixer to the main mixing board (or snake) as your keyboard-for-mains feed then the small mixer main output route to the main mixing board as your monitor mix. The output stage of your small mixer should have an EQ section that's separate from the per-channel EQ. Your per-channel EQ should be set flat to achieve as close as pass-through as possible. In this application, pre vs. post fade doesn't matter (AUX). I personally wouldn't want to have to manage this myself for the band.

An example of a mixer that can do this sort of thing is the ProFX4V2. Instead of "AUX" - replace this with "MON Send". There's a MON send that can use the GEQ while the main sends can be flat (no GEQ).

This doesn't let you do any panning tricks since the panning would be limited to "everything" using this board. If that was how you'd solve this - probably a larger board of your sound engineer would handle that without your having to take the reins.

It seems since ive introduced the modx 7 into my live rig that it competes too much with the guitar's and vocal frequencies in our in ear monitors. How do I rectify this issue? Korg, Roland and access music synths don't cause this issue at all!

Since your other synths don’t cause the problem, and certainly they are working with the same range of audible frequencies we humans deal with, I'd have to therefore, believe the cause is likely level related.

Try simply lowering the output Gain of the MODX.
Press [UTILITY] > “Settings” > “Audio I/O”
Change the Main L&R Output. Your choices are -6, 0, +6, +12dB

Sounds at the same frequencies can hide (mask) other sounds, you don’t mention how you’re handling the MODX as a Stereo Input. If Mono is what you’re delivering then that’s a lot of energy dead center. Not sure what the other keyboards mentioned are doing... but try to maintain the Stereo panorama in your earphones—this will provide more space for the guitars and vocal which have to be Mono. You have the option of using the stereo field, this will help give your Keyboard it’s own ‘space’ in your monitors.

Note that Audio I/O settings would be to adjust the gain in order to avoid clipping at the other end's input stage. If this gain stage is not distorting - then I'm not sure why, if a level issue, the provisions to change the level on the monitor mix would not be used at the monitor-mix area of the signal chain (on your monitor mixer board or combined monitor+FOH board). You could change your output level on Main - but may find yourself compensating with the main level slider and get back in the level danger-zone. If the upstream mixer doesn't have enough bells and whistles to do the job - then maybe output level would be how you handle it. Although I agree - and alluded to the possibility - that this may be level related - I'm not sure you're the best arbiter of the level by using the output gain stage - particularly if this is not to fix a distortion problem. Because it affects everything - I've got a feeling that somewhere else down the line your signal will be bumped up and you may end up back where you started.

The other option - using stereo field - is a good "other way" we both laid out. I wasn't clear - but the bias of your MODX in the stereo field allows for you to set the relative volume on your IEM box which would have a stereo pan knob, generally (and a mode for stereo). You wouldn't necessarily have to have a stereo feed if the pan position could be set on the upstream mixer which takes mono and pans it out to a stereo monitor mix.

I guess as a reference point, I haven't had any similar issues using the related Montage keyboard competing with guitar/vocals/etc. This would depend a lot on the upstream mixer and settings as well as your Performances. If you have custom Performances you built - they may not be well balanced themselves. Too many effects, all levels at 127, and other common "mistakes" could cook up some bad mojo. The presets should be fine - if you're using those then programming would not be a part of the issue. I'm not saying this is or is not - but something to throw out there as a possibility.