[Verizon Galaxy Note 4 Q&A, Help & Troubleshooting] bife2003:
I apologize in advance for the long read.

I recently activated Verizon service on a Note 4 that has been sitting unused since the Note 8 came out. The Note 4 is in perfect physical condition, with no scratches on the screen or body, and has been in a protective case since new. I did replace the battery in it prior to activation, as I expect for the life to have been deteriorated.

I did a factory reset and inserted my Verizon SIM card, and it worked perfectly. I installed my everyday apps and a few games on the phone, and I used it without any issue for about a week. During that time, it did install a couple of OTA updates, but I expected that since it had been "off service" for a few years.

After that first week, I put the SIM card back in my NOTE 8 for a couple of days, after which I went back to the Note 4. Initially, 4G data and cell service worked, but within the same day I started having service drops. If I was on the phone and a call dropped, it would show an error message saying "No SIM / SIM Error".

Her is what I have attempted so far:

First, I replaced the SIM with a new one. There was difference in the frequency of the random service drop.

When that did nothing, I assumed that one of the OTA updates from the prior week had done something to modem / radio. The phone was on 6.0.1 at that time. I unlocked the bootloader and installed TWRP, and first tried downgrading to stock 5.1.1, then to a "modified 5.1.1, then to CM 13, and now Lineage 15 (frequently factory reset / cache wipe. There has not been an improvement (or reduction) to the random service drop.

Yesterday, I happened to be on My Verizon online, and I had a message showing that Verizon no long had the information on my phone (IMEI), and requesting that I open a Chat Window to update my info. At this point, the Note 4 is still running the Lineage 15.1. The Verizon team member asked me for the IMEI, and when I looked under the settings, it said "None" under IMEI. In a quick search, I found a forum on Lineage 15.1 that said turning of the screen lock and rebooting the phone will reload the SIM and IMEI info. That worked, and I was able to view the IMEI and verify connection. I explained my issue with the Verizon support, and she said everything appeared to be good on her end. Over the course of 15 minutes, we tested my connection and it worked. About 30 minutes after that, it dropped service again.

Finally, here is where it is at now:

I start with no service. If I do nothing but restart the phone, it will show LTE service for 2 or 3 seconds, then nothing. If I remove and reinstall the SIM, I will get the same 2 or 3 seconds, then nothing. If I turn Airplane Mode on and off, that does not reconnect the service. If I change the lock screen security to "None" and restart the phone, there is no service, but if I set Screen Lock to "Swipe" and restart the phone, I will have LTE service for at least 30 - 45 minutes before it drops, sometimes a few hours. When it does drop, it does not come back on it's own. The only way I have found to get it back it to "Screen Lock - None, reboot, Screen Lock - swipe, reboot".

Will replacing the modem fix the problem? Is there a specific modem that I need to match up to the ROM? Thanks!