Mobile Hotspot Sign-In to Network "Error"

This began approximately 1 1/2 months ago where all of a sudden my device that was connected to my cell phone's mobile hotspot would all of a sudden stop working and a notification would appear at the top saying Sign into "network name".  When I click on the notification it takes me to what appears to be a website and it says "AT&T Free Message: Your attempt to connect to data service was unsuccessful.  Please try again later."  This was happening when connected to either my or my husband's cell WI-FI hotspot, which both have the same exact AT&T Prepaid Plan.  When it began it would only happen once a week at most, but it got more frequent as time went on.  Fast forward to April 28th I purchased an AT&T Mobile Hotspot device and also purchased a plan specifically for that device ($55 for 100GB per month) and this has begun happening more frequently on my cell WI-FI hotspot, my hotspot device, and my husbands cell WI-FI hotspot.  At this time I have had it happen twice within a 10 minute span.  There is nothing that helps other than time and possibly restarting the hotspot device or my cell phone.  Does anyone know why this is happening and what needs to be done to stop it?