Changing the DNS provider

[Inxsible]

Right, clear now. Internally.
You'd want to see where are your clients looking for their internal dns resolution and put an override there. Presumably it's Unbound. So try a host override.

Yeah, unbound is running in resolver mode and I already have a host override setup for adguard pointing to the IP of opnsense.

EDIT: Turned out that the SSL check was on in the Real Server configuration of HAProxy. Not sure how I missed that !!!! :angry: