Why not use a domain?
#WebSocket connection fails on remote conections
wintry tusk
With a domain you could change the IP the domain is pointing to locally
dusk lichen
Three routers. Interesting setup.
Two questions.
1 - Why do you hate yourself and want to cause yourself pain?
2 - Where is your client in this diagram?
dusk lichen
You're dependant on "ISP Router" doing hairpin NAT correctly, it sounds like that isn't working.
You might me able to get "Secondary Router" to DNAT the connection back to your server if it's a commercial router with advanced features.
Otherwise using a domain so you can override the hostnames locally is likely your best solution.