My Samsung Galaxy TabA SM-T555 has a few problems

I have the samsung galaxy tab A 9.7 inch, SM-T555. i have to connect using my Wifi , as the tablet doesn’t recognise the sim card, a big disappointment when i purchased the tablet,the seller said that it had been tested and the sim worked fine he said,so i just kept trying to find a solution. but finally given up on this. I get the Wifi signal going off and on always.I have noticed that there are a lot of people using a Samsung galaxy Tabs have got the same Wifi problem or similar.just wondered if anybody has any new solutions or ideas, I have tried the SIM in my phone and it is working I have tried gentle cleaning of the gold pieces,so it must be the SIM drawer that has a fault as the SIM card is working in other devices,ide be grateful of any help,Thanks,Regards Gillin.