Firstly, the strength of the Freeview signal at your aerial doesn't remain constant. It's affected by all sorts of things, such as the 11-year sunspot cycle, individual solar flare's on the sun's surface, atmospheric pressure, electrical storms, interference from other electrical devices in your home (or in your neighbours' homes), such as computers, monitors, TVs and printers [all of which produce RF energy]. So it's possible that your current problems may have absolutely nothing at all to do with the $G phone signals and could easily be solved by installing a signal booster (for about a tenner from most large supermarkets).
However, given that you received a postcard, it might be that it really is a 4G problem (which has only now come to light when the Freeview signal strength has been reduced by one or more of the factors I've mentioned above). So you need to start here:
https://at800.tv/