The router is serving as my Internet gateway and is supposed to also serve as my ISP's IPTV router.
In Openwrt-19.03 all works.
The MT7621 is using the newer DSA switch architecture.
My ISP provides Internet and IPTV services via VLAN 12.
My previous config is
option quickleave 1
option network wan
option zone wan
option direction upstream
list altnet 0.0.0.0/0
option network lan
option zone lan
option direction downstream
What I am trying to say is that pinning it on DSA does not sound viable, there are significant changes in kernel code between OpenWrt 19.x and Master branch, notwithstanding in OpenWrt's own code base (netifd | UCI | rcpd etc).
Your node may encounter a bug of sorts that needs fixing. But there is little to go on from your report, e.g. a crash log to start with, kernel | snapshot version.
Not very in line with the topic, but sort of, I have a similar issue with IGMP snooping crashing my DSA router with the QCA8K driver. Maybe these issues are related so I'll leave here a link to the crash log (the output of the serial connection):
On EdgeRouter X for example the interfaces are labeled eth0-eth4 by default, not wan, lan1-lan4. Master interface aka CPU is called dsa and ports are labeled like eth0@dsa, eth1@dsa, etc. So in a case of ERX eth0 refers to a physical port.
Here’s an example:
# ip -br link
lo UNKNOWN 00:00:00:00:00:00 <LOOPBACK,UP,LOWER_UP>
dsa UP ff:ff:ff:ff:ff:7a <BROADCAST,MULTICAST,UP,LOWER_UP>
eth0@dsa UP ff:ff:ff:ff:ff:7a <BROADCAST,MULTICAST,UP,LOWER_UP>
eth1@dsa LOWERLAYERDOWN ff:ff:ff:ff:ff:7b <NO-CARRIER,BROADCAST,MULTICAST,UP>
eth2@dsa UP ff:ff:ff:ff:ff:7c <BROADCAST,MULTICAST,UP,LOWER_UP>
eth3@dsa UP ff:ff:ff:ff:ff:7d <BROADCAST,MULTICAST,UP,LOWER_UP>
eth4@dsa DOWN ff:ff:ff:ff:ff:7e <BROADCAST,MULTICAST>
br-iso UP ff:ff:ff:ff:ff:19 <BROADCAST,MULTICAST,UP,LOWER_UP>
With DSA - there will be a master interface which wouldn’t be used in your /etc/config/network. The rest of the interfaces are “slave” interfaces - they are all individual interfaces unless added to a bridge. On your device it looks like there is a lan bridge br-lan with interfaces lan1 lan2 and a single unbridged wan interface which nevertheless is still a part of the master DSA interface.
If VLAN tagging doesn’t work as expected on WAN interface - you can consider adding all three interfaces to a single bridge, enable vlan filtering on that bridge, and add the vlans both to the bridge and to single interfaces. Last I’ve heard though IGMP snooping and bridge vlan filtering was causing kernel crashes with mt7621 + DSA. Not sure if this had been already patched tbh.
See this thread - it will have some good info relevant to your device.