@David-TP the server is reachable from the WAN side using either the IP or DDNS

It is not reachable inside the network using either WAN IP or DDNS

It is reachable inside the network using its internal IP 192.168.1.70

Note that the server is behind an Archer C80 running in access point mode.  But everything was working fine using the old crappy Vodafone modem/router I had prior to upgrading to the vr1200v, nothing internal changed on the network other than the modem/router.

just in case the previous message wasn't clear

(DSL) -- Vodafone modem -- Archer C80 -- server

worked fine

(DSL) -- vr1200v -- Archer C80 -- server

has problems

  @David-TP OK I have a relevant update.  I moved the server down to the office where the vr1200v is and it works properly when connected to that device.  I can hit it from inside the LAN either using the DDNS or the public IP.

I can't understand why it doesn't work when connected to the Archer C80 (when it used to work fine) but this does seem to indicate that the vr1200v is at least performing loopback.

Unfortunately it's not feasible to leave the server in that office because it's not my office :(  I will need to get it working from behind the Archer C80.  The C80 is in Access Point Mode w/DHCP off so I can't understand what the problem could be.  Any further help would be greatly appreciated.

The answers to your other questions are:

Is your current firmware Archer VR1200v-v1-build 20220415?

Yes.  The C80 also has the latest firmware 1.10.0 Build 210716 Rel.51301n(4555)

and is the modem provided by Vodafone or you purchase it by yourself?

bought it myself, dealing with Vodafone is like dealing with the phone company :)