Current ATT Home Base user

I live in rural upstate NY and I have a 2016 ATT Home Base Router with a very good data plan of 250 GB for 60.00 a month.  My Homebase router is recently sloooooowing down.   When I tried to upgrade to a new model,   ATT told me to go to my local "ATT store" and they could upgrade it.   The ATT store  said if I upgraded,  I would lose my data plan.  ATT stated the ATT Store didnt know what they were talking about as I was grandfathered in.  

MY question is,   Can I buy a unlocked NETGEAR NIGHTHAWK 1100,  insert the sim card (currently being used in my ATT HOME BASE),  into the Nighthawk and press on,  business as usual?  I hope this questions makes sense.   Also,  if any knows away to get a hold of ANYONE in ATT customer service who truly gives a damn,  please let me know.  I have never,  in 58 years, been treated so badly by a company.  There customer service is a joke......