As far as I recall the use of custom ports is supported in the standard install. I know there have been a few threads here in the community around the topic as well.
As far as for the OP, @Clare_Clever you may try to look into the application logs to see if that can give you some guidance to what went wrong a bit more in depth.
Otherwise I would initially suspect it may be some conflict due to your Nextinstall on the same host.
Do you know if you have another reserve proxy installed that handles Nextcloud apart from the built in nginx that comes with the Bitwarden install?
Otherwise I would also highly recommend reaching out to the official support as linked above, as they will be able to further support and assist you as well.