Mobile Data extremely slow or does not work.

I found this phone on the side of the road with a broken screen and two locked sims some time ago. I bought a new screen and made the replacement at ease. Upon figuring the phone had a lock code, I reset it to factory defaults. Unfortunately, I can't seem to get internet from mobile data on it. Wireless works perfectly fine.

I can find APN but it won't register to Network provider. Either it'll be stuck loading forever or it'll tell me there's an error and to try again later. Occasionally it does register but either I still get no internet connection or will have it but at a very, very, and I can't stress this enough, VERY slow speed. Let's say it takes about a minute for a random google search and a video won't even play. It also stops working altogether fairly quickly. Also tried connecting other devices through hotspot. Result: No internet at all on those devices.

Possibly worth noting is that's it's very hard to get the SIM 1 to work as the bracket that holds it together is a piece of crap and a special contraption is needed for it to stay in place. Some sim cards don't even work at all no matter how hard I push.

I don't believe I could've damaged the phone while replacing the screen. It was a very smooth procedure. The phone's screen was, however, completely cracked and the digitizer was dead as I found it, which means it must've suffered a very hard fall. It's lucky enough it wasn't run over by a car and crushed.

Would definitely appreciate any help and will be glad to provide more information.

Thanks.