Devices with webserver connected to IOT interface can no longer be accessed via hostname

In any case, you should not mix mDNS and DNS as they are completely different technologies.
It is not trivial to pass/resolve mDNS across subnets, instead you can set up static leases.
The hostnames configured with active static leases can be resolved from any downstream subnet.
If you also need to resolve inactive leases/hostnames, set up hostnames as well.