Please post your actual Nextcloud config (as noted in the support template and do so by using occ config:list system preferably since most of the Docker images used multiple config files + environment variable injection so just looking at config.php isn’t enough) as well as your Docker Compose.
Based on a few clues in the thread so far, don’t believe you’re actually using the AIO image/stack. Possiblly the community micro-services image, but there are other possibilities.
Also, can you clarify specifically what RP you’re using? Standard nginx does not have that UI. It appears to be one of the third party ones, possibly Synology’s.