Unnable to port forward. Connection Timed Out.

> Model: RAX120|Nighthawk AX12 12-Stream WiFi Router

   RAX120[v1] or RAX120v2?  Look for "Model" on the product label.
Firmware version?  Connected to what?

> [...] when I connect to my external ip [address] [...]

   "connect" from where?  A system on your LAN, or a system in the
outside world, or what?  And that address is...?  If you're worried
about revealing secrets, then the top half of the address, "a.b" out of
"a.b.c.d", would satisfy most of my curiosity.

      ADVANCED > ADVANCED Home : Internet Port : Internet IP Address

> [...] it says connection timed out [...]

   Hmmm.  The usual error from bad port forwarding is "connection
refused".  "connection timed out" suggests that communication was
established with the target server (so port forwarding worked), but the
server's responses were not getting back to the client.

   As an educational experiment, you might try specifying a bad port
number in a port-forwarding rule or a server address:port specification,
and seeing if that error message changes.

> [...] this setup has worked for me for the last two years but then
> suddenly stopped working [...]

   Firmware update?  Some other odd-ball change?

> [...] I have disabled all firewalls [...]

   With my weak psychic powers, I don't know what "all firewalls" means
here.  But that "connection timed out" complaint would be consistent
with a firewall blocking some traffic.

   No wireless "guest network" involved, I trust.

   For the usual problems with port forwarding, see:

      https://community.netgear.com/t5/x/x/m-p/1859106

   You seem to have covered much of the stuff there, but not all.