HTTPS Certificate error when using site to site VPN

First post! I apologize for any posting mistakes. Have a problem that I couldn’t google my way out of.
I set up remote access with duckdns works both locally and externally from my home. No problems. Working for almost a year LOVE HA and everyone that puts their priceless time into it!

First location: My home where HA server lives everything is groovy
Second location: Site-to-Site VPN to home from a VPN-capable router

I have a second location where I use a router to do site-to-site VPN. At the second location, I can ping my home 192.168.1.xx and I can connect to HA by hard typing https://192.168.1.xx:8123 which brings up a certificate error. This can be ignored and will connect to HA so VPN is working.

Problem: https://xxxxxx.duckdns.org:8123 doesn’t work at remote site, local address always throws a certificate error at remote site. At home and away from home everything works as expected.

Googled this up and down and tried a bunch of things still no luck. Any help discussing this and the area’s topics I should search for would be massively appreciated.