Freeradius with lets encrypt certificate

[André]

freeradius cloned from github:
https://github.com/FreeRADIUS/freeradius-server

Tue Dec 29 14:31:40 2020: tls - Failed verifying chain: error:1414C086:SSL
routines:ssl_build_cert_chain:certificate verify failed:Verify error:unable
to get issuer certificate
Tue Dec 29 14:31:40 2020: rlm_eap_ttls - Failed initializing SSL context
Tue Dec 29 14:31:40 2020:
/usr/local/freeradius/etc/raddb/mods-enabled/eap[1031]: Instantiation
failed for module "eap.ttls"

I'm using a let's encrypt certificate , but I'm getting this error message.

What should the files I should be using for the cert?

Best regards,