IPhone randomly saying invalid sim?

So I bought an iPhone 7 from someone I knew that was stolen. I paid 25 for it then spent 80 for a guy to jailbreak and remove the iCloud. He put something on the SIM card and told me not to take it out. The phone works perfectly but maybe after 2 hours it'll say invalid sim then I have to restart and it'll be back to normal. What should I do and I don't really have the money to get it checked out again.

Transferring phones?

SO heres the deal, I have this iPhone SE. Its fine for what it is, but it has only 16 gb of storage. That would also be fine, but 10 gb of my storage is taken up by "other" (not photos, music, vids, whatever) and I can't delete it. I want to just buy a 64 gb iPhone SE on eBay (same kind) for like 100 bucks (unlocked) and transfer my phone service onto the new one. Is that possible to do at home without risk? Or do I need to go to the retarded Verizon store? If its possible to do at home (I've heard swapping SIM cards does the trick), what is transferred and what isn't?

Why don't I get notifications when I get a missed call on iPhone?

Whenever someone calls me or facetimes me and I don't see it so it goes to missed call. But I'll go on my phone then and I won't get any notifications on the lock screen that I got a missed call. And there's no notification on the FaceTime app or phone app but in settings it says notifications are on. This happened on my old phone, then I got a new one and it was working for a little bit now I don't get notifications again. What can I do?