The PC that 3CX is installed is connected via cable.
Not asking for PBX but for others devices trying to be connected to with softphones
in extensions options did you untick "disallow use of extension outside the LAN"
how is it set in 3CX , public ip is set to dynamic?
for sure FQDN can't resolve if public ip has just changed, you need about 6 hours to DNS propagate if you are Pro license and 6 minutes if ENT license
try to put 3CX logs in verbose mode to have maximum details and see if you can catch something new to help fixing this
Perhaps i'm wrong but my feeling is not about 3CX but more related to network settings.
Other point , when you switch 3CX computers did you use a backup to restore on new one?
The Android Client automatically tries to use the internal IP and if not reachable it will then try connect to the external IP via FQDN (actually it will always try both but will connect to the one that answers first).
Have you tried using the tunnel mode to see if this helps? It requires Port 5090 open on the PBX side's firewall
My use is to make backup (with schdeduled backup) every weeks on a 6 weeks rotation for same computer saved on FTP server out of customer side, i'm almost able to follow what happens on backup and with 6 differents files i've a chance to have one working if a disaster comes.
something don't know if you already tried with android app
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.