richdocuments aka Nextcloud Office is a “connector” and works with different WOPI clients e.g. OOS and OnlyOffice (but definitely CODE/COOL is recommended)
from: https://apps.nextcloud.com/apps/richdocuments
This application can connect to a Collabora Online (or other) server (WOPI-like Client). Nextcloud is the WOPI Host.
unfortunately richdocuments readme on Github focus completely on Collabora which might show the direction more clear.
I would focus on this issue. Troubleshooting steps from Collabora integration guide should help with other WOPI clients as well. Most frequent is wrong routing of server to server requests e.g. routing to an IP address or internal interface rather public domain or reverse proxy (in order to get proper TLS connection).
@Borges please edit your post and add details as per required support template.