It’s my impression that there is a bug in Peplink with VLANs.
The bug manifests with some combination of (VLAN, UDP, DNS, Firewall Rules, Outbound Policy) but I’ve not been able to conclusively identify it.
You can read my experiences here: Outbound Policy + VLAN bug in 8.5 and 8.4.1 - #13 by soylentgreen
I’ve got a similar scenario, and think there are two issues:
- IOT device on a VLAN eventually loses DNS (DNS is typically a UDP transport)
- Other devices on the untagged LAN can’t contact the VLAN on UDP, even if UDP Relay/Multicast is enabled.
I mention this because, even if you are doing everything right with your configurations, it’s possible it may be failing due to one (or more) bugs…