I have my SIM800L running on 4V from a 2A-capable supply and connected to a 5V Arduino through a bidirectional level shifter.
If I put my Giffgaff SIM card from my iPhone in it, it connects. (Giffgaff runs on the O2 network. The LED flashes once per second for around 4 secs then reverts to once per 3 secs.)
But I want to use it with a free data SIM I get from Three. I registered this in a Three mobile data dongle and in that, it connects to the Internet and receives texts. (You have to top it up to send texts.) In the SIM800L it just flashes once a second for ever.
Is there a compatibility problem between the SIM800L and the Three network?