|It is possible to configure the existing router to act as a public accessable wireless router, but I would suggest to use a second router for that.|
If you will use only the existing router, anyone, connected to that one router, can see your private network and one may try to hack your system.
Therefore, if you're using a second wireless router, you can use an ip address range for the public network, which differs from your private IP range.
In that case, connect only the WAN port of the second router to one LAN port of the private router.
Assuming the first (existing router) has ip address 192.168.1.1.
Then configure the WAN port of the second router with ip address 192.168.1.2 / 255.255.255.252.
So the second router can connect to only 192.168.1.1, not to your other computers in your private network.
The internal network for the public router should be set to e.g. 172.16.0.1 / 255.255.0.0.
The original poster should always write the last response !!!
Let us know, if the problem is solved !!!