The site works on a computer and warns on a phone.
THE USUAL CAUSE
A missing intermediate certificate. Desktop browsers frequently have the intermediate cached from visiting another site, so they can complete the chain. Mobile browsers often cannot and fail.
THE FIX
Reinstall the certificate including the issuer's intermediate bundle, through cPanel > SSL/TLS > Manage SSL sites.
With AutoSSL, running it again usually resolves it, since it installs the full chain.
OTHER CAUSES
The phone's date and time are wrong An old Android version with an outdated trust store, which no longer trusts a currently valid root A mobile network proxy inspecting traffic An app using a different hostname from the website
CHECKING
An online SSL checker reports chain completeness and will tell you whether the intermediate is being served. That confirms the diagnosis before you change anything.
IF ONLY ONE PHONE IS AFFECTED
Check its date, and whether its operating system is very old. A device several major versions behind may not trust newer roots.
