There has been a long standing issue with handcrafted airports (which the WU5 airports provided by MS/Asobo qualify for) where sometimes the ILS frequencies will not show up in the ‘freq’ list in the aircraft.
Some developers set up their airports in such a way that these frequencies do show up, others don’t. It is partially down to how starred airports work within the sim and afaik developers have to manually re-enter the ILS frequencies as part of their ‘build’ for the airport for them to display in the avionics system in the aircraft. If they don’t do this then the frequencies will not appear in the aircraft avionics systems, even though the frequencies are selectable and useable.
I had a look at some of the handcrafted airports for WU5 and found that whichever developer was used for creating ESSA, EKRN and EVFA they did not do it this way and so the frequencies don’t show up. (I didn’t test other WU5 airports).
I tested one airport in WU6, EDDS and that airport’s frequencies did show up.
There isn’t any error with the GTN750 mod here and you may wish to see about creating a zendesk ticket for the specific handcrafted airports provided by MS/Asobo where this is occurring. A list of all handcrafted airports provided in the sim can be found here:
https://forums.flightsimulator.com/t/guide-handcrafted-airports/409633
(You could also search the forums to see if such a bug report has already been created for MS/Asobo handcrafted airports- I haven’t done this but off the top of my head don’t recall seeing one, if that helps.)
I also have the 3rd party EKCH and can confirm that they have not set up their airport in a way that allows the frequencies to be displayed.
For 3rd party hand crafted airports you would need to report the issue to them directly.
This particular issue has been around for months and some developers are just waiting for Asobo to fix this issue as it only affects “starred” airports in the sim and they take the view it is an Asobo issue to correct. Other developers use the workaround to ensure their customers are not disadvantaged in the interim by “programming” these frequencies themselves.