Im looking to confirm that the routers WAN IPv4 address is the 24.something address that FG sees.
Most port checkers like canyouseeme.org only check TCP. They wont check UDP.
Make sure you have the UDP rule in place, fire up FGU and choose LAN connection and see if you can get someone to connect to the 24 IP.
I cant at this time - at work - but if your players arent online you could pop into the FG discord and someone there will test it with you.

