MR6150 keeps searching for the network

I just received my MR6150. It worked great in the first few days, and I plugged it in on the desk to charge the battery last night. In the morning, it kept searching for the network, so I power cycled it, and about an hour later, it was still searching. Occasionally, it is connected and then starts searching again. The SIM card has no issue on my mobile.
Did anyone know what I could do to keep it connected?
Thanks, Marian.