Issues with displaying already connected Collabora Online docker server

Platform: Ubuntu 20.04.2
Nextcloud Version: 20.0.9

I have Nextcloud installed manually and everything’s been smooth so far, with the exception of trying to set up Collabora. Given this issue that I came across in trying to install the built-in CODE server, I opted to run a local docker container for Collabora instead. The group I’m running this for is small and the collaborative editing needs are limited, so the server resources should be optimal.

I started the container with docker run -t -d -p -e "domain=MY\.DOMAIN\.TLD" -e "username=admin" -e "password=S3cRet" --restart always collabora/codeas instructed here.. In the Admin settings on Nextcloud, I have pointed to the container, entering in the input box https://CONTAINER-IP:9980, with a successful output stating that the Collabora Online server is reachable.

Here’s the issue. On Firefox, opening up a spreadsheet (for example) will seem to load up: I can access the share menu and the doc details on the right, but the window where the spreadsheet should show up remains blank, with no indication of loading/errors. On Chrome, I get the same issue except instead of a blank space, I get the following error: https://CONTAINER-IP:9980/loleaflet/4aa2794/loleaflet.html?WOPISrc=https%3A%2F%2FMY.DOMAIN.TLD%2Findex.php%2Fapps%2Frichdocuments%2Fwopi%2Ffiles%2F4198_ocq15yumhnu7&title=New%20Spreadsheet.ods&lang=en&closebutton=1&revisionhistory=1.

I’m at a loss of where to look from here as I don’t have much experience in self-hosting larger stacks like Nextcloud (or Nextcloud itself). I found however in the logging section of the settings the following: Host was not connected to because it violates local access rules.