Thank you for reaching out to us in our community and welcome, sorry to hear you are facing issues with your Samsung J3 handset since switching to us from Three, are you able to try the Sim in another handset to see if the same occurs?
I understand it isn't an old phone and was working with your previous provider however this doesn't mean an issue hasn't occurred after or there are other issues with the phone not working correctly with the SIM, have you tried factory resetting the phone first, you can back up all the information Via your Google account first?
I appreciate that your friend's SIM does appear to be working in your handset, however from what you've said it does sound like a probable issue with your handset. This is due to the fact that you've mentioned that you were experiencing signal issues with the previous SIM and that your handset now seems to randomly acknowledge your new SIM card inserted at random times. This could indicate a possible issue with the phone SIM card reader.
Have you been able to check the SIM card tray and inside the SIM slot to see if there is any dust or anything that could be causing any issues?