First, and just for the sake of trying, try a hard reboot (press & hold Home Button and Power Button together for ~12 seconds then release. If the iPhone does not restart, press on Power button).
If rebooting doesn't change anything, look at this guide, step 31, right under the screw circled in red. There are two prongs that you can also see in step 32 of the same guide.
1/ Make sure they are there, in the shape and appearance that you see in step 32, and not bent, broken, or anything. If they're broken or missing, you will need to change the lightning connector using that same guide.
2/ If the prongs are there, connect the screen but do not snap it back in place. Then turn on the iPhone, go to settings>general>accessibility>assistive touch / turn it on. Then short the 2 prongs.
2-A/ If you are taken out of settings and back to home screen, it means the home button function is working fine, and you need to replace the home button flex cable attached to the screen using this guide.
2-B/ If you are not taken out of the settings App and back to the home screen, it means the home button function is not working properly.
2-B-i/ Disconnect then reseat the lightning connector from the logic board. If this doesn't help:
2-B-ii/ Examine the logic board between the battery and the sim card holder. If you see something missing/broken off, you could have a "prying" problem meaning something broke off during previous manipulation, and needs micro-soldering skills to be repaired. If you do not have the needed skill, we can point you towards someone with experience in this kind of repair.
2-B-iii/ It could be some other issue on the logic board => better try/exclude all the above before exploring in details what else could be wrong on the logic board.