My back button are not working

It's possible the connector for the buttons and lower speaker has come loose; does your sound also not work on the phone? If so, that would indicate an issue with the flex assembly that holds the buttons and speaker connector so it would be worth your time to try and reseat that connector. Here's a guide showing how to replace that assembly.

Manuales / Samsung Galaxy J7 (2016) / Flex front buttons and audio connector - Nadie Me Llama Gallina

I would suggest following the first three steps then skip forward to Step 5 where the second picture shows where the button/audio connector is. Unplug it and clean both sides with 90% or higher concentration isopropyl (rubbing) alcohol then reconnect it and see if it's working any better. If not, you may have to replace the flex cable as show in the complete guide.