My 7+ and my wife's 7 are both on AT&T, and reception has been out of whack since 10.0.3.
Before upgrading, I had a 6s+ and my wife had a 5s. At our house we used to get full bars of signal, and it was still fine when we upgraded to the 7+ and 7 all the way up to 10.0.2. But when 10.0.3 came out to help with the issues Verizon customers were having, it really messed up our service. Now at our house it sits at ONE bar, and sometimes two.
So Apple, trying to fix Verizon's issues totally messed up mine on AT&T!!! They didn't need to mess with the AT&T modem as much being as hardly anyone had issues with the new phones on AT&T. Verizon users seemed the most effective.
So just to be sure, I put my wife's simcard back into her old 5s which is still on 9.3.5. Sure enough, FULL bars. So needless to say I'm a bit disappointed by whatever Apple changed in the GSM baseband.