Why is my mobile data not working at times?

My iphone 6s plus mobile data sometimes work sometimes doesn’t work properly and i tried different sim provides but the issue is same my wifi is working fine but my mobile data sometimes doesn’t work at all and when it works it speed is very low sometimes when it doesn’t work i have to restart various times then it works please help me guys i have break my head alot into this but can’t do much did everything reset network settings & updated ios reset device did everything then too i am fade up with cellular thing please help me guys i really need your help