To be completely honest, given the changes and modifications you’ve already done, I’d suggest simply reinstalling FreePBX again.
The 20 minutes you’ll spend reinstalling and configuring the PBX will be quicker than the 3+ hours you might spend troubleshooting the issue.
If, after a clean install, you find phones are still unable to register, then cast your suspicions on the router, or more specifically the firewall within the router.