I see on the VPN set up page, you can add more than one rule, and each rule can have a different account, maybe you can try to set up different rules in order to get different accounts?

I know the router supports VPN Passthrough, but it seems there is no option to set up a VPN.

If the computer can get an IP address from the main router when it is connected to the access point wireless, you should have internet access. Maybe you can try to ping the IP address of the access...

That is V7. here is the link to the firmware: http://www.tp-link.com/en/support/download/?model=TL-WR841N&version=V7#tbl_j :)

I think you can check both if you are not sure which is one to use:)

Do you have any IP address on the computer?