I Have the following setup in one place:

We have a small local ISP through wireless. I have a external parabolic antenna, connected to a external usb wifi radio, connected through USB to a desktop old PC.

The pc connects do the ISP wiki network, then do a Dial Up (PPPoE) connection through the this wifi setup.

This will expand with others mobiles devices to be used. When I need, I take my home wireless router and connect though Ethernet in the PC, which is shares the internet.

The problem is that the PC must be always ON and working.

I would like to buy a wireless router which could be an AP to the mobile devices, notebooks, etc, as also could connect to the ISP Wifi/PPPoE network. So, this device must:

1. Have one radio with detachable antenna to connect to the external antenna. It must connect as client to a network and then dial up the PPP
2. Have another radio serving as AP (infrastructure) to the local place

This can't be very expensive. I found a candidate:

( http://www.tp-link.com/en/products/details/?categoryid=1682&model=TL-WR2543ND )

It have 3 deatachable antennas, working with dual band. Officially, his firmware doesn't support it.

My supposition: If internally there is 3 or 2 distinct wlan ports (like wlan0, wlan1), and there is support, i could use a OpenWRT, DD-WRT or Tomato to make this works. It also have 1 USB port, which I cold use to connect my actual USB Wifi card on it instead to the old PC.

Another alternative, is a router that can do this out of box, with the original firmware. But I don't think this is a easy thing to find.