N910F 6.0.1 Unable to register on network

For the past five days my N910F has been unable to register on a network. This problem first arose while I was using the Kyubi Note 5 ported ROM. Initially I was able to manually enter the APN details and use mobile data without issue, but had to keep manually re-entering the APN info as the phone would "refresh SIM data" every ten minutes. I could search for network operators and could see every available network in my country except my own, I was advised to attempt to connect to them all by my carrier's support but all would show as registered for a split second before dropping straight away.

The following day I flashed stock Marshmallow that I downloaded from Sammobile N910FXXU1DPD3 through Odin which factory reset the device, I've since rooted and put TWRP on it. Now when I search for network operators I can see my network along with the rest of them, but same thing happens and it can't properly connect.

The SIM card works in an old Galaxy S 3 and I've had my carrier send me a new SIM which arrived yesterday and after activating it, is doing the same thing.

When I bought the phone, I bought it second hand and it was carrier locked and the carrier would not unlock it. As a workaround, I went to a dodgy GSM shop who did something which essentially changed the IMEI of the device so that it would work, so my phone's IMEI when I dial #*06# and the one under the battery/on the box do not match. I only add this because I've read online that it could be an IMEI issue? However, this different IMEI hasn't caused me any problems for the last year and a half of owning the phone (apart from the fingerprint scanner not working).

Does anyone have any suggestion for the next step? Is it worth sending to Samsung for a warranty repair? Should I try a different modem?


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