I agree, if it is an embedded device specifically designed to connect to a limited set of services, there is no reason to expect it to follow DNS provided by DHCP.
Conceivably, there's no need to it to even use "real" DNS at all, you could just run a server that responds to queries like "updateserver.ecorp" and save the hassle of even announcing these servers to the public DNS at all.
Also, DoH or not, there are plenty of other ways to ensure that ads get through a DNS filter. For example, a local hosts file could be included in firmware updates and they would just need to make extra effort to ensure that the server IPs didn't change (an elastic IP or load balancer in AWS would be all you need, then it can persist even if the VM has to be deleted).
Conceivably, there's no need to it to even use "real" DNS at all, you could just run a server that responds to queries like "updateserver.ecorp" and save the hassle of even announcing these servers to the public DNS at all.
Also, DoH or not, there are plenty of other ways to ensure that ads get through a DNS filter. For example, a local hosts file could be included in firmware updates and they would just need to make extra effort to ensure that the server IPs didn't change (an elastic IP or load balancer in AWS would be all you need, then it can persist even if the VM has to be deleted).