When you press a button on the MX-450, does the red transmit light blink on the screen (upper right on screen)? If not, the remote isn't sending anything.

It could be the RFX-250 antenna module. Or it could be the MX-450. RF boards failing in URC remotes is a thing (especially if it has ever been dropped), the rest of the remote would still work fine.

It sounds like you are following the installation manual for troubleshooting. If you have done all those steps, then it does sound like you might have a hardware defect somewhere. Product substitution would be the next step but of course it requires you to have extra products.

I would go back over your remote settings one more time. Set the MRF-350 to ID 0, set the MX-450 to RF remote ID 0, make sure the other RF settings under 'RF Setup' are set to "All Devices Set Same" and "All Lines". Make sure when you press a button on the remote, the red transmit indicator is flashing on the remote.
If you do all that and you get no flashing RF light on the RFX-250, then it certainly sounds like something is defective.