Losing 2.4G internet connection

I use LTE routers to connect to 6 digital signs in different locations where there is no local wi-fi network.  I set routers up to stay on (no sleep), 2.4 broadcast only,and a 2 GB limitation per month to match up with my data plan.  For two months these worked fine.  Now 5 out of the 6 are emitting a wi-fi signal but no connection to the internet.  I have not exceeded data usage.  Bill is automatically paid.  Is there a set-up setting / process I have missed?