@RikkyH 

Hey

Im going to have to be honest with you and say what you are trying to achieve isn't going to work with that ISP Router.

The 901 is basically just an AP and therefore cannot support the VLAN creation, so what happens is it tags the VLAN to the traffic and chucks it to the router on VLAN1.       My guess is the router either 

1.  Sees the packet tagged for VLAN2 and ignores it unless the tag is for VLAN1 (default vlan)

2. Strips the tag and sends a reply on VLAN1 (default) as this is the only VLAN it knows, therefore the packet wont transverse to the other VLAN2 for guests and basically its lost.

I would say 2 is the more likely..

You will need a router or gateway that can support VLANs (tagged and native) to make this work..  this is outside of home grade stuff and into business, something like the TL-R605 

Better option might be to get a gateway router (WiFi Router) to replace the WA901 that supports broadcasting of a guest network and let it do the hard work for you.   Yes the IP the guests get will be on the same VLAN and IP Range, however the guest feature will restrict access between the guest and private SSIDs via ACL rules.