@kogan 

My ER8411 work with wireguard client from windows computer

I have alo a wireguard site to site between ER8411 and a Unifi UXG and this work to..

have you done any change on the config? new ip or somthing on router?

  @MR.S 

nope, nothing changed - but firmware 1.20.

As I described, sometimes very rarely, it starts to work. But only for short.

Problem exists on clients only, not for site-to-site tunnels

  @kogan 

Ok then I don't know, but very often strange things happen when tp-link upgrades the routers, I upgraded two ER707-M2 now and both sites went offline and I had to downgrade. after a lot of work and testing, I found out that I had to delete both server and client config for OpenVPN, then I was upgraded and the site did not go offline.

so there is always something with TP-Link routers. that's what makes it so exciting to work with, but you never know when a network goes down and you have hours and days of work 

You can of course try to contact support but do not have expectations that they can help, but try..

  @Clive_A 

It's in the wrong thread, but I'm commenting on what you write

a small comment regarding the ER707-M2. I had two OpenVPN servers configured, one with split and one without.
there was also some openvpn client configured. the strange thing was that all the Openvpn services were disabled but nevertheless the whole network crashed. all devices lost internet. so everything was dead. after hour with testing i figure out that when I deleted openvpn server and client everything worked.

afterwards I tested and created the server and client again and then it worked.

remember I adopted via WAN with no local controller