NAT Translation

Hi Team

My ISP has given static address x.x.x.17 on their side and x.x.x.18 /31 and Google DNS 8.8.8.8, I can ping the ISP x.x.x.17 from my router, when I source my LAN on 172.16.x.x /24 I could not, they dont route back to my LAN and they said use NAT overload so I did and IP NAT source list 172.16.x.x x.x.x.18 overload, the good thing is I can source from my LAN to WAN side of the ISP x.x.x.17 but my switches can not ping the WAN link on the ISP side, I do have ip default-gateway on the switches pointing to my LAN interface on the router. NEED HELP