if its never worked it might be a mono issue (there were some previous issue with its root repository certificates - ie the ones it trusts)
its possible is just doesnt like that digicert root, or cloudflare interim certs
in which case this may be useful (it says the issue is that mono doesnt trust the digicert baltimore cert)
https://www.mono-project.com/docs/faq/security/
according to that you may be able to add that digicert root certificate into the mono trust store yourself - but im not sure how that would go with DSM. but you may want to just stick with the http variant for a while
if you were running v3 you could disable certificate validation - it still uses the encryption it just doesnt care if its valid or not (sort of like TLS)