Question: Why am I able to access LAN website from WAN?

Sorry if my terminology is not on point with this question, I am pretty new with playing/learning about networking. Here is my situation...

Hugesnet service without static ipv4 address, there is a LAN server with a web app used within the LAN with no problem. There was interest in being able to access the web app from outside the LAN (want to be able to access the web app from home). After playing around with port forwarding and learning more about how the internet works I came to the conclusion that without the ISP giving me a static ipv4 address this route would not be practical. With nothing left to lose I started playing around with ipv6 (no idea what I was trying to do, just messing around). From outside the LAN I entered the ipv6 address for the LAN server into the browser and I was able to access the web app from outside the LAN. This seems to work for a while (week or two) then stops, I believe it has something to do with DNS, after I reboot the modem it starts working again. Any idea why with ipv6 I am able to access the LAN server over WAN? Thanks for any input!!!