Unable to access PMS via mobile/desktop application locally

[turtleinarock]

Server Version#: 1.41.5.9522
Player Version#: 10.26.0.2578

Hi, I have my PMS server in a docker container. It is accessible only via a reverse proxy (SWAG) on port 443. I am able to access PMS from the mobile app or the web just fine on any external networks via plex.mydomain.com. I can also access PMS from plex.mydomain.com via the web browser on my LAN. The issue arises when I try to access PMS from the mobile or desktop application on my LAN. Ex: I am using my cellphone to test.. As soon as I switch to a VPN service or my LTE data, PMS loads up instantly. However, when I try to access PMS, on my phone, via the application, it just loads and loads and says the server is offline/unavailable. However, if I access PMS via the web browser on my phone without switching networks, it loads up instantly.

I have tried reinstalling the app, rebooting my phone, rebooting the router, toggling using insecure connections (on phone), and setting a manual server connection (in plex mobile app, on phone).

Does anyone have any ideas what might be happening here? I would be glad to provide any additional information

[turtleinarock]

TL;DR:
Can access via:
plex.mydomain.com on WAN
Plex mobile app on WAN
plex.mydomain.com via web browser on LAN

Can NOT access via:
Plex mobile app on LAN

My leading suspect right now is this because I am using ISP provided modem/router gateway:

https://support.plex.tv/articles/206225077-how-to-use-secure-server-connections/?mobile_site=true#dnsrebinding

[tom80H]

Could be a DNS configuration on the router.
The browser on the client could be using a separate DNS (or a dedicated config on the computer).

[turtleinarock]

I plugged in my dd-wrt router behind the ISP modem (which I was trying to avoid because it cuts my download speed in half) which solved the issue. Definitely some sort of DNS configuration on ATTs router. As soon as I pointed to common DNS (Cloudflare, Google) the issue was resolved instantly.