Hello !
I was not able to find Archer C1200. Therefore I selected some other AC model.

I am having problem with one particular router (Archer C1200) connecting to a work VPN (L2TP/IPSec). It was working before. I have recently connected an AC1200 Wi-Fi Range extender with AC Passthrough RE365(EU v1.0). I suspect that after that some ports may be blocked preventing me from connecting to the work VPN. Is there a way to check the rooter Ports in use by the rooter and reset them. I have reset the router by pressing the reset button for 10 sec., and the Range Extender, updated the firmware version (current: 2.0.3 Build 20211219 Rel. 67401 (EU)), and I am still having the same problem.

Currently the Range Extender is off. If I use an older TP-Link Router or my mobile phone as hotspot I am able to connect to the desired VPN. Therefore I am pretty sure the router blocks the VPN connection. All Passthroug-Options of the Archer C1200 are enabled as they are on the old router, with which the connection is possible. 

Any help would be appreciated. Thanks in advance!