I'm using the Archer C9 as a wifi router at home, to replace the bad wifi provided by my internet provider's VDSL box (Proximus).

When working at home, I need to set up a VPN (L2TP tunnel) between my laptop and my company, so I enabled PPTP / L2TP / IPsec Passthrough in the NAT Forwarding/ALG settings on the Archer C9.

This works fine, but after a few hours the VPN suddenly stops working (no more traffic going through). 

I have to reboot the Archer, re-establish the VPN connection and then I can work again.

As you can imagine, this is very annoying.

Now after a lot of trying several settings and looking on the internet for people with similar issues, I finally discovered the problems were caused by NAT Boost on the Archer.

When I disable the NAT Boost (in Advanced -> System Tools -> System Parameters), my VPN connection works flawlessly for days on a row.

Is this something normal that can be explained, perhaps it is a bug in the firmware ?

Also, I hope this helps other people when facing a similar issue.