Good Network connection but cant access internet

I have an IMAC running 10.5.8 and I use a Sierra Wireless AT&T USBConnect Lightning Air Card for my network connection. Its been working great for 6 months or more.

Starting a few days ago it connects fine to the AT&T switch, verified by an AT&T techinician. I can see the outgoing and incoming bytes increasing as normal and I get a DHCP IP address assigned by AT&T each time I connect.

But when I open Safari half the url turns blue and it just sits like that for a minute or and then I get no network available. Email and pinging from the MAC ping utility both fail also.

Reboots, reinstalling the AT&T s/w, and replacing the AT&T device didn't work. Has anyone seen this issue before?

Moved to correct forum.

Welcome to the Mac Forums. It would help if you gave the specific model of the Air Card since Sierra has several. The one that I did find on their site was a model 305 which apparently is not supported in OS X - at least there were no drivers for it other than Windows.

You say it was working for 6 months so I have to assume the model you have is supported in OS X. If it stopped working suddenly and you changed it out for another unit and have the same problem, then it may be something that you recently installed or updated that's interfering.

Yes, I'm sure its something that's changed on the IMAC so I've completed steps 1 - 5 of the "Basic OS X/Mac Troubleshooting" ideas posted by D3v1L80Y and will most likely continue with steps 6 - 10 if I don't get something better soon.

I believe the device is a 302 but I know the device is working fine and has been workiing fine until a few days ago.

Does anyone know how I can maybe delete Network completely and reinstall network because it has to be something has got corrupted in MAC network?

I'm really close to just reinstalling the entire OS but I would hate to go that route. But this is my sole home computer that I relie on heavily so I can't continue without much longer.

Please help