Nighthawk M2: Losing cell connection

Hey,

I'm using a Nighthawk M2 as a primary router at home (since 2 months). I have removed the battery and the router is connected to external power all the time.

In the last few days I have started noticing a strange behaviour: The router looses cell connection all the time (approx. every 10 minutes) and then takes up to 1 minute to connect again. This is very odd, because it worked fine for the last 2 months. What could be the cause of this? I have already tried to change settings (changed from auto to manual cell selection and so on) but nothing helped.

Could it be possible that the router needs a battery to work properly?

Thanks in advance
jr