Custom DNS entry issue

[BrutusBoots]

So I connect to a remote office using wireguard in opnSense and it works great.  In this office there is a main system with the ip 192.168.2.100.  This system was setup with a hostname of 'mainserver' with several services running on it with names like 'wiki.mainserver', 'test.mainserver', etc, etc.  This remote network provides no DNS.  The users simply enter these names into their hosts files and everything work.  Not the best setup but I have no control over it.

How can I setup unbound with the following entries so when I connect via wireguard I can simply use these names and have everything work?

mainserver 192.168.2.100
wiki.mainserver 192.168.2.100
test.mainserver 192.168.2.100

I've tried adding a host override with 'mainserver' as the domain but it doesn't seem to work.  if I use 'mainserver.test' it will work.. making me think that it's not possible with a single word domain?

any advice?

Thanks!