I had a similar issue with a public game server that required connecting through the WAN even if clients were local on the LAN. I considered split DNS (resolving the name differently depending on the source) but it was complicated for my setup. Instead I found a one-line solution on my OpenBSD router:

    pass in on $lan_if inet proto tcp to (egress) port 12345 rdr-to 192.168.1.10

It basically says "pass packets from the LAN interface towards the WAN (egress) on the game port and redirect the traffic to the local game server". The local client doesn't know anything happened, it just worked.