...

$ cat /etc/resolf.conf

# This file was automatically generated by WSL. To stop automatic generation of this file, add the following entry to /etc/wsl.conf:
# [network]
# generateResolvConf = false
nameserver 172.17.192.161

1. cd /etc 

2. echo "[network]" | sudo tee wsl.conf 

3. echo "generateResolvConf = false" | sudo tee -a wsl.conf

4.  wsl --terminate Ubuntu  #(Terminate WSL in Windows cmd, in case is Ubuntu not Debian).

5. cd ~/../../etc (go to etc folder in WSL).

6. sudo rm -Rf resolv.conf (Delete the resolv.conf file).

6.5 Connect to your vpn

7. In windows cmd, with the vpn connected do:

  > ipconfig /all to get the dns primary and secondary.

135.5.25.53
135.239.25.53

8. With the dns primary and secondary . Replace X.X.X.X in the folllowing steps with the primary and secondary DNS ips

9. echo "nameserver X.X.X.X" | sudo tee resolv.conf (Create resolv.conf and append the line.)

10. echo "nameserver X.X.X.X" | sudo tee -a resolv.conf (Append the line in resolv.conf)

11. wsl --terminate Ubuntu (Terminate WSL in Windows cmd, in case is Ubuntu not Debian).

12. sudo chattr +i /etc/resolv.conf

13. And finally in windows powershell with Administrative rights:

Get-NetAdapter | Where-Object {$_.InterfaceDescription -Match "Cisco AnyConnect"} | Set-NetIPInterface -InterfaceMetric 6000

References