Things get more bizzare!

I hooked up my factory reset 605 running 2.3.1, running it totally in standalone mode

Default MTU of 1500 - all VPNs work to Omada and Draytek Gateways (pure IPsec only VPNs though)

WAN MTU of 1352 - optimal for my ISP and was broken on ER8411 - all vpns work properly

Remote ER605 running 2.3.1 pure IPsec dial in VPNs - working on all WAN MTU

Remote ER605 running 2.3.1 L2TP VPNs - not working with any WAN MTU, at either end

I think we have 2 issues

ER605 2.3.1 - broken L2TP VPN MTU size

ER8411 1.3.3/1.3.6 - WAN MTU settings effect VPNs on clients

  @GRL 

Well, I'm a little confused here now, I'm behind a UX7 from Unifi, so it, like the ER8411 and ER605, can't connect, but I set up an L2TP server on an ER706w which also has an MTU of 1380. but I can connect to it, I can't connect to the ER707-M2 with an MTU of 1380. so I don't really understand what's going on.
I have disabled SD-WAN and all other VPNs on the routers I'm testing with.
I think I'll wait until you're done with your test :-)

but I think an MTU of 1400 would have been a better choice for the Omada routers L2TP Server

L2TP to a ER706W

High strangeness indeed!

I think its safe to say that ER605 2.3.1 has a broken L2TP implementation though, definitely something wrong with it...

ER8411 has a Broken WAN MTU implementation since i dont see the same issue on ER605....

  @GRL 

I'm sure there is something. But for me it's not a problem. I stopped using L2TP many years ago, but it might be good to report our findings to the Omada team.