I have a 2860n+ connecting to vdsl on WAN1 and using the wifi mode of WAN2 to connect to 4G via a Netgear Aircard 785 Mobile Hotspot.

The failover mode set in Load Balance/Route Policy works very well if I test it by dropping the vdsl connection. However every few hours the wifi connection between the 2860 and the Aircard is dropped, for no apparent reason, and takes a few minutes to re-establish itself - which it does with no intervention from me.

Clearly, a "failover" system which might be down when the main connection fails is not a very satisfactory system!

The Aircard is about a foot away from the router. I can see nothing in the settings of either which could cause this behaviour. Does anyone have any ideas as to what might be occurring?

Is the mobile hotspot on a fixed channel or does it change its channel? If it's changing its channel, the router would need to change its own wireless channel to keep up, which requires scanning then reconnecting, so there would be some delay incurred.
If the delay is longer than a minute or two, I do think contacting DrayTek support to see if they can help would be the ideal thing to do.

The hotspot is on a fixed channel, and I have tried the connection both using dhcp from the hotspot and with a static IP address with no difference. I've been logging the WAN connections and there does not seem to be any pattern to the failure. Sometimes WAN2 will stay up for hours and sometimes only for a few minutes. If I connect to the hotspot directly from a wifi PC the connection stays up.
I'm running an AP902 in an adjacent room and tried turning off the 2.4GHz wifi on that but that made no difference.

Looks like a line to support coming up.....

Strange, it's quite reliable for me when I use the feature with a Nexus 5X phone on 2.4GHz. I do think contacting support would be best.

I had a useful exchange of emails with Draytek support - including a preview of the new 3.8.2.4 firmware (which I think had the wrong modem code for me, but interesting anyway).

Further to their suggestion I set up an iPad mini as a hotspot and connected it to WAN2. The connection was stable – implicating the Aircard 785S as the source of the problem.

I had previously attempted to upgrade the firmware on the Aircard using a file downloaded from the Netgear site, but it had always failed. By more diligent searching on the net I discovered that it was necessary to install a special driver, obtainable elsewhere on the Netgear site but not mentioned in their firmware upgrade instructions! I installed this and lo and behold the firmware upgrade now worked.

The WAN2 connection to the Aircard now appears to be stable – result!

Another issue I had was that if I leave “Load Balance” ticked on in the WAN2 settings (as shown in the Draytek guide) I am unable to access any of my external IP’s (IP cameras etc) from the LAN, even though the open ports settings to those devices specify that connection should be through WAN1. Unticking the load balance box solved that problem.