Look, many of us have the modem mounted on top of or next to the transceiver and suffer no problems. If I were you, I would not listen to worry warts until you give it a go yourself. Try it out with a temporary installation
... if you have no feedback or other problems, fix it permanently in place. Damn the "experts", full speed ahead!
IF problems do arise, buy some cip-on ferrites and start hanging them on the cables
, monitoring the problem as you go. Likely this will cure things.
Honestly, it is awful easy to over-think and over-worry about such matters. A little experimentation will answer a lot of questions.
good luck, mate.