Try another port to connect thru. Try one between 49152 - 65535 (eg: 64049) as these are usually the very last ISP's block. ie: go to LW's menu bar, Tools>Options>Advanced>Firewalls & change the connection & listening port both to the higher port. If you have manually port forwarded your connection device then you'll need to do another for the new port. Also adjust any firewall settings if they specify ports. Remember you should only have one active software firewall.

Did you get a manual with your NextNet device? Within it should be instructions on how to log in & how to open certain ports or how to enable UPnP. Both the Linksys & NextNet devices will have internal NAT firewalls. Also check the wrt54g note under the UPnP section here Connection problems? Check here first! (click on link)