In our office we are currently have 4 BT Business Infinity Premium lines installed. 2 of the routers have started dropping out on multiple occasions. I reported one of these two lines to BT who noticed a fault on the line and sent an OpenReach engineer out. They fixed the line, done a 5minute test and advised that this was working again at 100%. Since the engineers fixed the line yesterday morning the router has rebooted almost 80 times!
The other two lines have had no issues or dropouts and the only difference I can see is that the two faulty routers have the firmware version 4.7.5.1.83.8.256 (Type A) on and the two that work have version 4.7.5.1.83.8.252 (Type A). The two that work haven't had a firmware update since November 2017 and I'm worried that when they do update they'll start rebooting every 20mins too.