Gmail hacked regularly

Hi everyone,

I use a Chromebook and have a Gmail account. I've noticed someone is accessing my gmail account this past week (I can see it in the activity log, that someone with a different IP address has been ''piggy-backing'' off me somehow. Every time sign in to Gmail, this unknown user seems to log in within a minute of my login. I immediately click on ''sign out all other users'' icon and I have changed my password numerous times and I have two-step verification.

Despite changing my password, using 2-step and a VPN, this same pattern occurs. Interestingly the IP address of the other user is very similar to mine - only one digit is different. I foolishly have been using a public/shared wifi - as the downstairs city council office gives free wifi. So I suspect the other user is also in my apartment block as their IP is similar to mine and corresponds with the range of IP addresses hosted by the city council downstairs.

It's clearly not safe, even with a VPN, so I'll not use the public wifi any more, however the hacker is able to access my account from other networks when I'm out also. They do seem to be only able to log in immediately when I have, and I can throw them out immediately, but is there a way I can stop them? The usual ''change password and 2-step verification'' advice hasn't worked. Thank you for any advice you may have, I'm new to chromebooks but have been using gmail for many years.