I want to add that the relay server is never handling any customer secrets (so a low value target), and we have techniques in place to reduce the probability of DoS (increase the cost to the attacker).

So even if someone would be able to break into the server through the small attack surface, he would not be able to change any setting on any of our customer's devices. Or even read any status either. Of course, if someone can break into our server, the DoS is inevitable, but so far this never happened.