Remote Desktop Connection triggers "A revocation check could not be performed for the certificate" from systems not part of Active Directory Domain

At my firm, we have various remote workers who utilise a combination of L2TP/IPSec VPN and MS Remote Desktop Connection to connect directly to their workstation onsite. In the wake of the coronavirus implications, I am having to expand to our full (generally less technically minded) workforce very quickly, so want to get rid of a unhelpful error message on connecting.

Some quick facts:

1) I have a AD integrated Certificate Server, already correctly configured to issue certificates appropriate for use in the Remote Desktop setup
2) I have applied the appropriate settings through Group Policy to apply the Certificate Template, and also things like enforcing SSL, etc.
3) Connections are straight from the RDC client to the workstation, no brokers or anything like that.
4) Users connecting in from home are NOT part of the Active Directory Domain.

When a user tries to connect in, they get the following warning:

"A revocation check could not be performed for the certificate". If they ignore this, the connection is then successful.

On closer inspection of the Certificate being served up to the RDC, the CRL Distribution Point they are being served is based on LDAP. As they are not part of the AD Domain, my feeling is that this LDAP location is probably inaccessible due to a lack of valid authentication.

I'm aware I can change settings on the CA server properties, but I'm weary of doing this as I do not want to break our domain in any way. What would be ideal would be for someone to talk me through exactly what I can do to make an http based CRL Distribution point and set it as the priority served up to RDC clients when they try and connect from outside the domain, so they can check for Certificate Revocation without needing AD authentication.

Many thanks in advance :-)