A test was done for the M9 in which the LAN was changed to 192.168.x.1 with the subnet of 255.255.252.0 which worked fine now.

An important thing to check is whether the WAN IP/DNS and the LAN IP and subnet to be set are in the same subnet or there is a subnet inclusion/overlapping relationship. If this is the case, the LAN IP cannot be modified successfully.

@Tony

The router has been holding its own for the most part & only had to change the LAN IP address once lately.

You mentioned changing LAN to 192.168.x.1 with the subnet of 255.255.252.0 worked fine. And that also works fine for me when using LAN to 192.168.1.1.

Sometimes (but not always) when the router resets itself to the default LAN 192.168.0.1 it also changes the WAN Subnet Mask to 255.255.0.0. And this prevents me from changing the LAN IP back to 192.168.1.1 which I need.

There does not appear to be any way to reset the WAN Subnet Mask to the default 255.255.255.0, other than doing a router reset which wipes out all my settings.

One time I took the router out of the system & let it sit for a day with the power on. When I came back the WAN Subnet Mask was back to 255.255.255.0, But I'm always able to do that.

How do I manually reset the WAN Subnet Mask to the default 255.255.255.0?

@StymiedToo 

If the Deco WAN interface is DHCP, you can't. If you know what IP you want I would set the WAN for static in that case.

@Tony 

I tried using a static IP address but to no avail. But I'm not really sure what I'm doing.

What makes the WAN Subnet Mask flip to 255.255.0.0 in the first place?

What makes it eventually change back to the 255.255.255.0 default? If something is triggering it to return to the default then why can't I access the same "trigger"?

Yesterday the WAN Subnet Mask flipped to 255.255.0.0 and now I can't use the router. It's powered off & just sitting. Hopefully when I turn it back on in a day or two the WAN Subnet Mask will somehow be back to the default.

I let the router sit overnight and turned off. When I turned it back on the following day the WAN Subnet Mask was reset to the default 255.255.255.0.

The setting can obviously be changed but only when the router feels like doing it. This is not acceptable. If I'm not home to remove the router from the circuit then no one can use any device that uses the router.

I've had several routers over the years & none of them just changed settings on their own & consequently shut down my system. That's really a crappy design.

Goodbye TP-Link.