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...

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...

@MR.S WAN MTU of 1400 seems to be the middle ground for me - I can connect to Omada and Draytek gateway VPNs properly with traffic passing, and RDP isnt borked What happens if you change the WAN MTU...

@MR.S I will see how my 605 behaves today and let you know!

Overall, although I identified the MTU being the issue on my setup, it leaves me with a conundrum. At 1500 (default) my VPNs work, but due to packet fragmentation RDP to a server i use all the time...

@MR.S Do you get the same effect as i was - VPN connects, devices pingable, but zero TCP/UDP traffic to and from them?