If the batteries and connections are definitely okay, I’d check the receiver antenna next. Make sure it isn’t damaged, sharply bent, trapped against metal or carbon, or buried inside the model. Try repositioning it and then do a slow range test in an open area.
I’d also test whether the problem happens only in one location. If the range improves somewhere else, interference could be involved. If it still drops at roughly the same distance everywhere, I’d suspect the receiver or antenna rather than the environment.
Another useful check is receiver voltage under load, since a brief voltage drop can sometimes look like a radio signal problem. I wouldn’t replace the transmitter immediately. A controlled range test with another known-good receiver could help identify the faulty component.
Once everything is working again, I’d probably take a break from troubleshooting and play Meccha Chameleon 2 for a while.