It can not increase the volume

@lualayokmalueth could be a bad speaker or could be a bad power/volume flex. I would start by replacing the volume flex. Since iFixit does not yet have a specific guide for this, use something like https://www.youtube.com/watch?v=NGy9AEbM... It'll be the same as for your 5G. Flex cables are readily available at many online stores.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this. Repair is War on Entropy!