I don't know if this will help anyone but damn is the documentation on how to do this kind of thing scarce so here I am. I was quite a struggle both learning all this and implementing it but it's been fun overall.
FreeDNS doesn't seem to have a way to add any kind of SSL Certs as far as I can tell. Not that I needed to, but when I tried explaining my situation to the hosting provider they suggested that the cert needed to be on the DDNS server.
I also couldn't get the Asustor Reverse Proxy to work either. Many of the tutorials and user suggestions i ran across suggested that this is the way to get SSL but when I tried to implement it I kept landing at my portal login and nothing was getting proxy'd. The netstat command was showing that whatever the reverse proxy port needed was not open - I also ran across this in a 2020+ thread on Asustor.
Right now I'll also need to manually update the certs and go through this process again in 3 months time. There might be a way to automate this process but it's a lot of steps that I'm not sure can be automated in this seeming (to me) flawed setup. In any case, it's been challenging but fun and I think in the next 6 months I'll upgrade out of Asustor and into a bigger server with a core Linux setup. I know nothing about Linux :)