[Off topic] problem with cable modem/router/reverse proxy setup

Completely off topic, but with so many bright brains around, perhaps someone would be able to help…
So, recently my system developed strange issue; I can access all of my home systems using FQDN from public internet, but when using the same domain name from LAN it does not work. Everything was fine untill recent change in my network (upgraded speed of connection, that required to replace router/modem from ISP).
Few words about my configuration:

  • I use cable modem/router from ISP (UPC branded Compal device). it is configured as router, with all firewall options switched off and DMZ configured to WAN1 IP of my router.
  • I cannect my router (dual WAN Draytek Vigor 2925) to this modem using 2 connections: router WAN1 to Connect Box LAN1 and router WAN2 to Connect Box LAN2. I use such double connection because of VPN configured on router and redirecting all VPN traffic to WAN2. This way I can monitor VPN traffic via SNMP separately from direct Internet connection.
  • On my LAN I have Synology NAS with reverse proxy configured. All trafffic on port 80 is redirected from router to NAS (open port), so it can be handled by reverse proxy. So it looks like:
WAN guest -> Connect Box -> router -> reverse proxy -> target system
  • Above configuration works perfectly fine, when accessing my systems from Internet. E.g. when accessing my web mail server (using mail.domainname.com), traffic is redirected correclty to proper server in LAN.
  • When trying to access same server from LAN using same domain name I get no connection… seems like timeout. Since I’m not even reaching the standard Synology WebStation web page, I assume in this case traffic is stuck somewhere before reaching NAS… Since I can ping Connect Box from outside, but I cannot ping it from inside using its public IP, I’d assume connection is disrupted on loopback on Connect Box (I can ping WAN ports of router and LAN port of Connect box without issues)?

In majority of cases I can survive with this setup, connecting using either FQDN or local IP, but for home assistant, in particular, with several other systems web pages embedded into iframes it is very annoying, since depending on configuration these are visible orly from inside or otside of LAN :frowning: