My setup is a 2866 (192.168.2.1) with DSL for WAN1 and a Zyxel 7490-M904 LTE router (192.168.3.1) connected to WAN2. Right now it's connected in a double-NAT arrangement, and WAN2 is seen as having an IP address of 192.168.3.2 by the Draytek, as the LTE router is assigning 192.168.3.2 to the Draytek via static DHCP.

The issue is with WAN2; sometimes the Draytek reports the WAN2 is dropped, , and comes back a short time later.

I've tried various different WAN Connection Detection settings. I would have expected Always On to have worked as expected, but this too drops the WAN2 sometimes. When this happens, I can see on the LTE router's own admin page that the LTE connection has not dropped, the up time is still increasing. This is further verified by running a ping from an external device to the LTE router's public IP address, and observing that the ping isn't interrupted. I can safely say that the LTE connection is stable during these times.

So then I'm thinking, why would the 2866 see a lost connection? I can only think of something with the cabling. However, what's double odd, is that as an additional test, I am running a constant ping from within the Draytek's LAN to the LTE router (192.168.3.1, the gateway of WAN2 as seen by the LAN) and this too continues uninterrupted.

So all I can think of is the Draytek is incorrectly seeing the WAN2 as dropped at the physical level?

I'm not even sure if there are any negative consequences of this "drop" - if the pings continue, then presumably other types of packets would too?

Any reason why you're not using bridge mode. That would get rid of double nat and perhaps also solve the dropped connection.

I had the same issue in IP Passthrough Mode (as it's called on the ZyXel).

I switched to double NAT to rule out it being a problem with that mode.

Replaced the 2866 with a 2962 and it’s been rock solid ever since.

*shrug*