Self-hosted installation: cannot login with android app

Hi,
I have installed bitwarden on premises.
Everything works fine: I can login via browser (web) and firefox plugin.
The only problem I have is with the Android app.
The error is “There is a problem connecting to the server”.
In the device logs I see an “Untrusted chain” error.
I am using a wildcard certificate from Let’s encrypt that works well with all my websites, imaps, smtps, vpn, etc.
The certificate chain is:
DST Root CAX3
Let’s Encrypt Authority X3
*.derinaldis.com
I also tried to install the CA and the intermediate certificate on the device, but I did not solve.
Do you know I can solve the issue?
TIA.
Cheers,
Andrea