"Not registered on network" after failed update

[Android Central Question]

I have written on another site, but haven't gotten answers yet, so I am posting it here as well.

Hello everyone,
I'm a desperate noobie here trying to fix this issue for three days now.

So as stated above I have a Samsung Core Duo (SM-G335H) and three days ago I connected trough SmartSwitch and it offered to update my phone. I agreed and during the middle/second half of it my laptop and phone just crashed. I revived both and SmartSwitch told me about the update failure and I made an Emergency Recovery. After that recovery, at first, everything seemed to be working normally, but on the next day I noticed: "Not registered on network " error. It recognizes the SIM card, but when you try to find a network it says "Can't connect. Try later." and so on. I can't call, other peeps can't call me.

I have been researching this matter for days now and am very upset about this. Some say stuff like Odin/flashing/third party tools/settings, I don't know what to do, as I don't have much experience with those things.

Btw, my IMEI is not null, it is being displayed (so I guess the EPF folder is not damaged). The APN settings (I'm on T-mobile) are still there, I don't know what to fix about them.
I am running 4.4.2 KitKat and my Security Patch Level is dated 2016 if that is of any relevance...
I also did a hard reset: nothing. SIM card works in my other phone, so it's not damaged. I just now saw that the end symbols in my Baseband version and my Build number don't match. Could that be it? I've seen people recommending updating manually/flashing with a modem file. How do I go about this?
Also: my phone is rooted. Only that I very clumsily attempted it months ago and before this matter here I would have problems with binary updates and what not, "phone is not rooted completely", I already fixed that issue though, it has SuperSu on it.
Anyway, I may have messed up somewhere along the way leading to this problem.

Please don't recommend me reseting/rebooting the phone again or just re-inserting the SIM card, that doesn't fix it.
I welcome any other suggestions, I hope somebody can help me.

 

[Rukbat]

Updating a rooted phone usually causes problems - the update is supposed to "fail" when it sees something not the way it's supposed to be, and before it starts actually updating anything. Considering that the phone came out almost 4 years ago, and isn't likely to be updated past the 2016 security patch, SmartSwitch shouldn't have even found an update.

If your carrier won't reflash the ROM, you'll have to do it yourself, but it's not difficult (just deadly boring) - see [Samsung] How to flash Stock ROM via ODIN.

But either way, you doing it or your carrier doing it, back everything up first! Flashing the ROM wipes all your data and all your installed apps. See Backing up an Android Device.