Netgear Nighthawk M2 brickware

I bought this router as part of an EE deal with a monthly rate about 2 years ago. With help from EE the router has now been unlocked, as the contract has ended. However, it's impossible to get the router to run with any SIM card from any provider. Not even an EE pay-as-you go card will work. Shouldn't this type of router be easy to operate? Why is it so difficult for the router to recognize the SIM card and provider an simply connect? I'm about to throw the router away and buy TPLINK instead. Resetting the router didn't help either. WLAN works but the data card is not recognized.