This is a strange one

I have several clients connecting through our router on port 22, port forwarded then to an internal SSH server.
I can log in ok to all accounts remotely except for one client who can only connect to the Draytek via domain name and NOT via the static public ip address.
Our DNS points 8 total public ip addresses of which one is the router's static ip. I cant think why they can connect via domain name and not directly to the router's static ip.
Anyone have any idea why?

Guessing on this, but... Could it be that the SSH client is doing validation of the SSH certificate presented by the internal SSH server? Perhaps their client is doing validiation and other clients aren't?