Vaultwarden, Synology NAS, certificates

Hello everyone, I would need some help, if you can.
I installed Vaultwarden on my Synology 224+ NAS and I would like to reach it without exposing it to the internet, so without using DDNS. I configured the reverse proxy and I have no problems connecting (via VPN) to the Vaultwarden server via browser (both from PC and phone) but when I try to connect the Bitwarden app from an Android phone or PC, my problems begin.
I think I haven’t quite understood the issue of certificates. Can anyone help me?
For the sake of completeness, the VPN is managed by my Fritz!Box router.
Thanks