Mwan3 stops working when ISP modems swapped

I have a NETGEAR router WNDR3700 with OpenWRT v18.06.2 installed.

It's configured with mwan3 to load-balance my 2 internet connections.

I have connected the first ISP Modem/Router (Modem A) to the regular WAN port ("wan" interface) and the second one (Modem B) with the wanb interface (LAN-1 physical port). Both the ISP modems are having DHCP enabled. All works well till now, with "wan" and "wanb" interfaces functioning perfectly fine distributing the load between the two ISP routers/modems.

But when I swap these connections, i.e connecting the first Modem (Modem A) to "wanb" and the second one (Modem B) to "wan", I see that the wanb interface is disabled and non-functional. It makes the connection in the regular wan port (wan) only functional.

What could be the problem? Any help is appreciated.

It may be because the ISP sees a changed MAC address and refuses to serve you.

Are these two modems from the same ISP or different services?


Its the same service provider (ISP), but one is FTTH connection and the other one is ADSL.

This is very likely the cause.

One of my two ISPs does this. If I have a non-expired DHCP lease, I can't change the MAC address of the modem, otherwise it refuses to serve me.

Try overriding the MAC address under Interface-->Wan-->Advanced Settings so that you keep the same MAC address for each interface regardless of which device is connected