I can explain how I do it but I use Apache on an Ubuntu 20.04 system as my webserver - this is 192.168.1.2. It has the LetsEncrypt certs.
OMV is on 192.168.1.3 and doesn't face the web directly.
I run SyncThing on OMV and on the Ubuntu/Apache device I have a virtual server as follows:
ProxyPass /syncthing-omv/ http://192.168.1.3:8384/
<Location /syncthing-omv/>
ProxyPassReverse http://192.168.1.3:8384/
Require all granted
</Location>
On that device I have a folder /var/www/html/syncthing-omv with a one line index.html file:
<meta http-equiv="Refresh" content="5; url=http://192.168.1.3:8384/">
So I'm away from home and want to look at SyncThing on OMV:
https://my.duckdns.org/syncthing-omv
Apache directs the request to the OMV device and the certificate is good.
Thanslate this principle to Niginx and you'll have it going.
But...this took a lot of working out, maybe isn't the best way, and other web apps such as Jellyfin required a different approach.