[Galaxy Note5 Q&A, Help & Troubleshooting] droseofc:
I've recently seen several threads from quite a few people who have the issue of having no signal, no recognition of sim card, unknown baseband/imei etc., and rebooting every minute or so after turning on.

Its happened to me 3 times now but the last two were fixed as another user had found out how to fix it so ill share his discovery that I used for this issue successfully. Ive posted the steps on a handful of different threads by users in need but hopefully this will save the extra threads and duplicate comments by myself as its hard for me to not comment with help when i am able to do so.

The exact cause of the issue is not known to my knowledge. It happens after flashing something. Can be supersu, a kernel or even a rom, and when booting after the flash you find you have no signal bars or even a spot for them, if your quick you can see under phone status through the settings menu it shows unknown baseband, unknown imei, everything that looks important unknown and the inevitable reboot, my system uptime was 1 min 05 sec each reboot. I tried flashing a rom which still had the same time of reboot but appeared as a bootloop as it never fully booted the new rom and rebooting preventing it from fully setting up...until....

1. Bottom line im 99% sure is to let your phone completely die. Others just so happened to be a certain way during this such as having the sim card removed and/or being in download mode. I followed the exact steps the user did, remove sim card, let phone die, reinsert sim card, charge a little and reboot. It worked. I had sim card out and was in download mode as one user had his sim out and another was in download mode when they fixed theirs. I do see the importance of being in download mode only because my phone has died before but i was able to boot to download mode a few times before it was truly out of juice. Seems as if letting it die resets whatever it is that is causing the phone not to read the information. Let it completely die, be sure to either be in download mode or boot to it after it dies until you cant anymore. Really doesnt hurt to remove sim as well but id bet that it is overshadowed by completely running battery out which then I am sure the phone wouldnt know if it even had a battery let alone a sim card or that it is a phone to begin with.

2. After the battery is gone completely, reinsert sim if removed, and charge it before booting, but just a little maybe 10%, just in case you have to let it die again if the issue is still there. Download mode takes forever to drain battery, the constant reboot drains pretty well and if you are desperate you can make it vibrate between each reboot by switching vol up/down vibrate-silent, fast enough and it almost vibrates nonstop for each reboot. I let it get down to 3%, then booted to download mode to think about what its done and die. Then I waited a few minutes, plugged in charger, booted up and was good. I had flashed a kernel, then got issue, had fixed it by this method, but su had disappeared so i flashed supersu and got the issue right back, so dont charge alot before knowing your for sure good.

3. I just realized im terrible at making steps or howtos and have lost the plan i had imagined for these steps.

BASIC EASY FIX- LET PHONE DIE, CHARGE PHONE TO 10%+, REBOOT PHONE. ISSUE SHOULD BE FIXED. OVERLY DETAILED STEPS ARE ABOVE TO PREVENT ANY CONFUSION OR QUESTIONS FROM THE TECHNICAL USERS AS WELL AS THE USERS, AT LEAST APPEARING, TO NOT HAVE ANY TECHNICAL EXPERIENCE AT ALL. (which is ok)

Tired of posting over and over across forum on new threads having the same issue. Hopefully it directs you here and if not idc ive done my deed for this topic and as hard as it will be for me to do i am not repeating on any future new threads. Theres been quite a few which is not a terrible thing but just detracts from the forum in general. Also this isnt even my fix, i found it same way you should have but credits go to a s6 thread by whoever that user was in their q&a forum.