Can't get Docker install running with Port 80 blocked

I’m very new to Docker, but I’m migrating my Nextcloud install from a Snap install because I couldn’t seem to solve my SSL issues with Snap.

My ISP (Altice One) blocks port 80 so getting LetsEncrypt working is a huge pain (especially for someone like myself who doesn’t really touch networking).

In any case, I installed a fresh OS (Ubuntu 18.04) and downloaded the files here to the server. I modified them accordingly (in docker-compose-yml I put my domain behind VIRTUAL_HOST and LETSENCRYPT_HOST, my email behind LETSENCRYPT_EMAIL, made up a root password for MYSQL_ROOT_PASSWORD. In db.ev I made up a password for MYSQL_PASSWORD. I then ran docker-compose build --pull followed by docker-compose up -d, but I get an “Internal Server Error” when I load my domain.

So I tried changing the port in the YML from 80:80 to 85:80 hoping that the bridge would bypass the port 80 restriction, but that seems to have made things worse. I can’t even get to the site now and I’m at a loss.

One thing that seems noteworthy is that when running docker-compose build --pull it skips LetsEncrypt because it’s “an image” for some reason

Any help is much appreciated.