the referenced “green mark” in Nextcloud Office settings only shows the connection form Nextcloud to Collabora works, it doesn’t show reverse connection neither client to Collabora… see this post
As I see you point your Nextcloud to https://nextcloudoffice:9980 - which is almost never reachable from the client… I would recommend you to use real public domain name and setup valid TLS certificates for both Nextcloud and Collabora… and point Nextcloud to the public DNS of Collabora (details of WOPI protocoll referenced here)