I am at my wits’ ends with a network problem.

The topology:

We have a new fiber modem for our new ISP (ISPnew) with 4 LAN ports (LAN_new), and 2 wifi networks, Wifi2GHz_new and Wifi5GHz_new.

  • My computer, Windows 10, usually connects via Wifi5GHz_new. Internet works, everything appears to be OK. Wifi2GHz_new works exactly the same.
  • My cellphone also connects to Wifi5GHz_new
  • I have a Debian server with SSH connected to LAN_new at
  • For testing purposes, I also have a RaspberryPi with SSH which I can connect to Wifi5GHz_new at or to LAN_new at

My computer cannot connect to the Debian server on LAN_new (not on SSH or any of the webservers that it is running). My cellphone, my girlfriends computer and apparently everyone else on the network can connect to the Debian server just fine.

So some experimentation showed:

  • Computer (1.3) can connect to the Pi (1.15) if the Pi is in Wifi5GHz_new. This works regardless whether computer is in Wifi5GHz_new, Wifi2GHz_new, or connected to LAN.
  • Computer (1.3) cannot connect to the Pi (1.16) if the Pi is in LAN_new and the Computer is in Wifi5GHz or Wifi2GHz.
  • If I hook the computer up to a LAN port, it connects fine to Pi 1.16 and Debian server 1.14.
  • All other devices have no problem connecting to Debian@LAN, Pi@LAN or Pi@Wifi.

In summary, Computer 1.3 on Wifi can only connect to devices that are on Wifi, and apparently not to devices that are on LAN. All other devices have no problem communicating to each other. This has probably nothing to do with DebianServer, Pi or the ISP modem/router, but with my computer.

Totally separate, the modem of the old ISP (cable, ISPold) is still active for another month or so, with 4 LAN (LAN_old) ports and a Wifi2GHz_old. On the old setup, PC ( via Wifi2GHz_old in that setup) can communicate just fine with Debian ( via LAN_old in that setup), this is reproducible even after switching everything to new setup and back to old setup.

So there is some setting somewhere in the routing of my PC or in the WLAN settings that somehow blocks going across the gateway, but it doesn’t even make any sense, since the PC (I think) has no way of knowing whether the target device is connected to LAN_new or Wifi_new? Any help?

One peculiarity about my PC is that I have 2 VPN installed for two employers, one Cisco AnyConnect and one GlobalProtect. Another peculiarity is that I took the harddrive of a completely set-up ThinkPad T470s and put it into a ThinkPad T490, then updated all drivers accordingly; but everything including Bluetooth seems to be working fine. Also, the ISP change happened much more recently than the computer HD switch, so the new modem has never even seen the original T470s.


Somehow activating and then deactivating AP isolation on the modem/router fixed things. Not that it should have, since it was deactivated in the first place, and for everyone else this was never a problem to start with.

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