Samsung A Series Phone is Not Connecting to the Internet

My Samsung A Series phone is not connecting to the internet, and it's been frustrating me to no end. I've tried everything, from resetting the factory settings to testing the SIM card on another phone, and the problem still persists. When I connect to Wi-Fi, everything seems fine, but when I try to use my mobile data, I get constant error messages or the internet speed is so slow that it's barely usable. It's not my SIM card because I've tested it on other devices, and the problem seems to be with my phone. Has anyone else experienced this issue, and if so, do you have any solutions? I'm at my wit's end and can't keep dealing with this issue any longer.