@Virgo

I'm using the router only to have redundant internet in my home office, therefore a reither simple setup, one WAN port is connected to a FritzBox (that one is capable of bridge mode), the other one is connected to a LTE router (that one is not capable of bridge mode). I therefore have a local network on each WAN port:

My current guess is (and I might have read that somewhere) that the router determines the online status by pinging the gateway of the WAN port. When there is any provider issue the LTE box or FritzBox remains of course online and pingable, therefor the port is online as long as these boxes run. Whenever I disconnect Internet on the LTE router, the port status remains online and active connections just fail. 

By following the config guide linked in my original question it seems I can change the online detection to something that should work for my setup.

I'm starting to think as well that this configuration isn't possible at all in controller mode, which would be disappointing for a product that claims to be compatible.

@Virgo 

I did this test from my PC listening to online music and browsing. Disconnected the LTE line, ER605 WAN port remained "online" but music streaming stopped after some time and I was not able to access web sites anymore. One I reconnected LTE everything restarted working. I guessed my PC was one "active connection" being routed using the LTE connected WAN port and since the WAN port remained online (ER605 view) nothing was "transferred" to the other WAN port. I only have very basic network skills TBH.

Pinging in that situation probably would have failed as well.

Still hoping someone pops up and can tell me how to do this, but mentally preparing to just rebring the router to standalone mode. Controller statistics are nice to have for my micro network, but the failover / WAN redundancy was the key requirement I bought this thing for...

@Felix81 

So....

I took the router out of controller mode, it reseted to factory settings, I configured the first WAN / LAN port to WAN, this was the only configuration I performed.

Disconnected the LTE line and again the port remained online and online. Reconfigured online detection (it was auto per default):

Disconnected the LTE line and within 1-2 minutes:

So I strongly suspect when in controller mode, this setting is always on "auto" and there's no way to configure it (at least no way I found)... I'm keeping the router in standalone mode for now, hoping this gets addressed.

@Felix81 

Maybe I'll give it a shot re-importing the router having that setting disabled and hoping the current manual config (resolving DNS servers) persists within the configuration...

@Felix81 

So, I did the test I talked about in my last post:

1. Validated that "online detection" config is still set to resolve Google DNS within the router
2. Disconnected LTE behind WAN / LAN 1, after short time port went offline
3. Reconnected LTE behind WAN / LAN 1, after short time port went online again
4. Disabled "WAN Settings Override"
5. "Acknowledged" the router within Omada Controller
6. I was able to validate that indeed the settings of the router were imported because the "Online Detection Interval" changed compared to previous config within the controller (changed from 1 minute to 2 minutes which fits the time from point 2 and 3)
7. Disconnected LTE behind WAN / LAN 1, waited for five minutes, WAN / LAN 1 remained online

So that didn't work out, apparently the custom DNS resolving online detection was kind of "overriden"