Setup for AT&T router

Customer has a Motorola NVG510 router that was installed by AT&T when they upgraded them to Uverse.  They are a small office, but they keep getting errors about invalid IPs, and they run out of IP addresses.  They have had AT&T get online with them in the past, only to say "it is fixed" when it clearly is not.

I stopped by today and they asked me to look at it.  I am no expert--which is why I am asking these questions today--but I saw where the router was configured for "Private" addresses only, in a range of something like 109.281.143.166 - .170, and those addresses are indeed what was showing on their PCs when I did an IPCONFIG.  So, I changed the router settings to utilize what they call "Public" addresses, and I set a more typical range of 192.168.1.2 - .250.  I saved the settings and reset the router.  After rebooting each computer, I was getting IP addresses of 192.68.1.x on each computer, and they could see their shares across the LAN just fine.

But the problem comes into play with internet access.  We could get on yahoo just fine--maybe because they are tied to AT&T??--but any other web page would either fail, or delay for the longest time, and even if we got on their main page, we could not browse to other links on the site.  In addition, their email software would no longer send or receive.

The customer says that in the past it has taken them hours to get a person on the phone at AT&T, so they had me set it back to those five "Private" IP addresses in the router, and now all is working like it used to.  But while I'm here, I'm wondering if this "rings a bell" for anyone, where you could suggest setup steps I could try in order to make it all work right using a full range of 192.168.1.x addresses.  TIA