UPDATE
Just returned from trip to local AT&T store....
Guy at this store was great and very helpful.
Turns out issue is not about the SIM card.....it's all about the IMEI number used.
Unfortunately, since PC does not have AT&T sell their phones, they don't have an IMEI number that identifies the device as a Cosmo. Therefore, the system doesn't know to provide it with 4G and mobile data.
After trying all the possible APN's available and not getting 4G, he entered an IMEI from an iPhone and Voila! the Cosmo got 4G and mobile data instantly.
So, for those of you that got your Cosmo's working without all the headaches I went through, the AT&T guy said there were two options.
One, you transferred an existing SIM card from a phone that is recognized in AT&T system / IMEI.
Or if you went to AT&T store and got new SIM, the person helping you did what my guy did and didn't tell you.
AT&T guy said that I need to see if it still shows 4G after about 36 hours from now, as he said their system does a periodic sweep of devices on system. He thought there might be a small chance that system might detect that IMEI is for iPhone while device is not an iPhone. If that happens, it would likely downgrade Cosmo back to 3G. And if that happens, he said to come back to store or call AT&T to reset it again.
So it goes......but at least I now have 4G and mobile data......
Thanks to all who helped. Really made a big difference!