After experimenting more, I noticed, that when WAN is set to "passthrough", then LAN can also be set to passthrough and LAN clients get IPv6 addresses. This is good, but not complete: now the ER605...

How IPv6 DHCP relaying can be configured to ER605? My operator LTE modem gives IPv6 addresses to my local network by passing through IPv6 DHCP information. For example, here are addresses for one...

I found the solution, to use different SNMP browser. I downloaded the ManageEngine's free MibBrowser tool, it was able to interpret properly TP-Link's MIBs.

@ASN45 Were you able to maintain VLAN but disable the another IP address? I'm asking as I have the same problem.

I also tried directly with POE mib, no luck. <obsolete information removed>

@Hank21 Thanks for this information. Before asking this, I already downloaded the MIB definition there and tried to read it with snmpwalk command. Anyway, there is nothing PoE related on the snmpwalk...