IPhone 6 Cell antenna works intermittently
The battery is right up against the antenna and some delicate ribbon connectors that connect to it. It is possible that in the swapping of your battery you might have mis-seated it (at best) or damaged something (at worst).

Did the antenna stop working after you changed the battery and that led you to bring it to a shop? Or was it already getting flaky before you opened it up?

Given that you said that it only worked for about 15 minutes, it is possible that it is either slightly out of place or the ribbon is damaged and the heat of the device is cause expansion/contraction which is causing the antenna to fail; i.e. after a few minutes of using the phone it heats up enough that the connection breaks. No guarantee that cooling it down would reestablish that faulty connection.

Assuming you're comfortable opening the device given that you've already attempted the battery, you could look at loosening and then carefully tightening the antenna screws and see if that helps anything. Step 14 on the iFixit teardown guide gives a closeup of the antenna.

The inside of the phone is delicate and very sensitive to static electricity, things getting bumped, etc. Sure, they're always around static electricity but they're designed to mostly keep that stuff outside of the case. Regardless, given that you mention intermittent behavior you might be able to get away with wiggling / reseating the antenna to bring it back to life. Just make sure to be properly grounded first!