I completed Taking Independence, the radio transmitter was working fine. Now I'm working on Old Guns and I can't get the radio transmitter to work. I haven't changed anything since completing Taking Independence so the transmitter should be working. Also, I can't assign a settler to the transmitter. Is Bethesda going to fix this? This is frustrating beyond belief!!

Check the wiring.
Check any switches, if any, in the wiring as well.
See if the local grid is overloaded.
Half that place is already wired when one marches in, so if you connect your generator(s) properly, it all comes on line, even the large water purifier.

Local gird is overloaded? How do you do that?