Failover to USB cuts web access if custom DNS set in DHCP LAN config

[Yakumounr]

Router Model Affected
Models: DSL-AX82U

Firmware Version Affected
3004.388.10_2-gnuton
3004.388.11_1-gnuton0_alpha1
3004.388.11_1-gnuton0_beta1

Is this bug present in upstream Merlin releases too?
Unknown

Describe the bug
On failover to USB only the DSL-AX82U can resolve domains (checked via network tools in webui and SSH) but no LAN clients can so they lose web access. I found a quick workaround in the Web UI then later worked out that this is happening as the DSL-AX82U route to the custom DNS servers is left indefinitely pointing to the Primary WAN on the failover -

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.54.86   0.0.0.0         UG    0      0        0 usb0
1.0.0.1         192.168.2.1     255.255.255.255 UGH   1      0        0 eth4
1.1.1.1         192.168.2.1     255.255.255.255 UGH   1      0        0 eth4

The workarounds I found in the order I discovered them:

1. Via Web UI -> LAN -> DHCP Server:
   Change Gateway from blank field to router's LAN IP or vice versa (ie. I would toggle blank and 192.168.1.1 each time USB lost connection), hit apply.
   You must change some field even if the results are the same or hitting apply doesn't process.
   This temporarily puts the DSL-AX2U offline interrupting all clients closing all connections.

2. service restart_net
   Similar to 1. this of course temporarily disconnects the router.

3. ip route del 1.0.0.1;ip route del 1.1.1.1
   Manually deleting the non working route entries via SSH instantly resolves the problem without disconnections.

Removing the primary WAN would prevent this problem but then it couldn't failback to primary when it's connection problems are resolved.

The only way I've found so far to have a smooth failover transition without having to log into the router (or scripting a quickfix) is to not use custom DNS settings for the LAN (nb. WAN config lets you set custom DNS for DSL or WAN but not for USB).

To Reproduce
Steps to reproduce the behaviour:

Dual WAN enabled.
Primary WAN port connected to a device that is powered but no internet connection (eg. phone line cut)
Secondary USB port connected to a USB tethered device (Android phone).
LAN - DHCP Server - DNS Server 1 and 2: set to custom DNS (eg. 1.1.1.1.1 , 1.0.0.1)
LAN - DHCP Server - Lease time: default (86400)
LAN - DHCP Server - Default Gateway: either blank or set to the dsl-ax82s IP (eg. 192.168.1.1)
LAN - DNS Director is off.
LAN - Route: Static routes are not enabled.

• Disable then re-enable connection on USB tethered device, or cause the tethered device to temporarily lose signal for long enough to trigger failback (move to dead spot or just enable Airplane Mode).
• Rebooting the router will also result in failover to USB with the bad route left, leaving an unsuspecting user to think their USB tether has failed.

Expected behaviour
Route to LAN configured DNS servers should be corrected on the failover and failback so LAN devices can access the internet.