Assuming you already checked such basics as:
Checking whether same SIM card work in other device in the same location
Router have good signal strength
Router running on latest firmware version
Correct APN is entered
Scanning for operators to check whether operator is available in your area
To proceed with investigating the issue I would need router's logs.
Please download router's troubleshoot file via WebUI (System -> Administration -> Troubleshoot) once o2 SIM card is inserted and send it to me via Private Message to proceed.