mobile

Hey all. I'm currently running a D855V (Australian model) rooted on stock Android 4.4.2.

So I recently upgraded to Lollipop but couldn't use it fully due to issues with some other software of mine. So I used the KDZ method to flash back to 10E. Since then I've lost all 4G/LTE on my phone.
I've tried changing the networking mode inside the mobile network settings from GSM/WCDMA to GSM/WCDMA/LTE but once selected my entire mobile network will drop. From there it will regain network connectivity for about 10 seconds (minus any data connection) and then drop off again, showing no network. It will continue to do this until I change the network type back to GSM/WCDMA or reboot (which defaults the connectivity method back to GSM/WCDMA anyway). I tried flashing the 10C firmware too (for Australia) but this will do the exact same thing.
I've also tried changing the LTE RAT settings inside the hidden menu to GSM/WCDMA/LTE and it still won't connect. It will drop the network like the previous method but will then reconnect only with a HSDPA connection rather than 4G/LTE.
I've checked the SIM card in another phone and it works fine with 4G/LTE. I've also checked my IMEI and it shows that I do have one so I can rule that out. Right now I"m at a loss. I'm thinking I need to flash a new radio or baseband but I can't find any info about that from any of the other threads I've read.

Any info at this stage would be great as it sucks having the 2nd best 4G speeds in the world and not being able to use them.

Cheers all.