[ONE General] MNNR:
Hey there! This is my first thread about fixing a phone's problem, I hope it helps some OnePlus One users, Okay, So the issue is that that the internet isn't reachable, Even if you can see the mobile data signal indicator (H+ or LTE). I was able to fix my friends' phone by updating the modem, And updating the OS from Cyanogen Mod 13, to the official Lineage OS 16 Rom, And to do that, Just follow my instructions, But first, You're obviously gonna have to download some files:)

Note:Please do it at your own risk, I'm not responsible if you bricked your phone, Still I'm sure if you follow the instructions correctly, nothing bad will happen:D

The required files

OnePlus One USB drivers Download

ADB and Fastboot Installer From this thread

TWRP Recovery Download

Modem files that I used Download I Found them on this thread

Lineage OS 16 Official zip file Download

Open GAPPS Download (make sure you select ARM, not ARM 64 from the download section)

Note: Unlocking the bootloader will wipe all data from your device (Including apps, pictures, settings, And everything , So make sure you backup all the important files.

Now follow the instructions

Make sure your phone is charged enough (50% or so) to get you through the whole process.

Install the USB drivers, And the ADB and Fastboot (You can find how to do this on the original Thread

Now you have to unlock the bootloader of the phone, To do that: Go to the main directory of the ADB that you installed, Then press SHIFT on the keyboard and right click with the mouse, And "Open command window here", Now Turn your phone off and boot it to Fastboot mode by pressing Vol+ and Power button until it say Fastboot mode, Now plug the phone to the computer, And type in the cmd:

Fastboot devices hit Enter (Your phone's serial number should show up there if the drivers were installed correctly).

Fastboot oem unlock hit Enter (wait for the process to finish).

Fastboot reboot hit enter (To reboot the phone to the system)

Next, You need to install TWRP recovery, but before you proceed, While the phone is turned on, Go to Settings>About phone, And scroll down to " Build number" and click on it 7 times in a row, Until it says "developer mode is on", Now press back button and press on developer options from the settings menu, Look for (CM Recovery Protection) and disable it, Now turn the phone off, And boot it to Fastboot mode again and plug it to the computer, On the computer, Open a command window in ADB's main directory again.

Type in CMD:

Fastboot flash recovery C:/User/where you've put The TWRP-3.3...... .img file hit enter (note: after typing " Fastboot flash recovery(space)" instead of typing the whole path to the TWRP file, you can just locate the file and drag it to the command window, And it'll automatically write the path to the file.

That's it, You can unplug the phone , Now Boot it to brecovery by pressing vol- and power button (as soon as you see the OnePlus one boot logo, Remove your hand from the power and vol- button).

Once you're in the recovery, don't touch anything, Just swipe right, Now, Take a backup of the current modem partition (just in case something worse than the internet issue comes up), To do that: In the recovery, Go to Advanced and click on Terminal, Write these commands:

dd if=/dev/block/mmcblk0p10 of=/sdcard/modemst1.bin bs=512 (press enter)
dd if=/dev/block/mmcblk0p11 of=/sdcard/modemst2.bin bs=512 (press enter)

Note:Make sure to go back to the main menu and backup the current ROM too (in case something goes wrong or you don't like the Lineage OS)

Now that you've backed up everything, You need to flash the new modem to the phone, So extract the zip file that you had downloaded, It countians 8 files, Now again, Open the command window in ADB directory, Boot the phone to Fastboot mode and plug it in, And Use these commands to flash the modem files:

fastboot flash modem NON-HLOS.bin
fastboot flash sbl1 sbl1.mbn
fastboot flash dbi sdi.mbn
fastboot flash rpm rpm.mbn
fastboot flash aboot emmc_appsboot.mbn
fastboot flash tz tz.mbn
fastboot flash LOGO logo.bin
fastboot flash oppostanvbk static_nvbk.bin

Here is an example:

fastboot flash modem (And drag the file"NON-HLOS.bin" here to automatically generate the path to the file)

Repeat the process with each file, When you're done flashing those files, Type Fastboot reboot, You need to transfer the Lineage OS 16 ROM zip file, And the Open Gapps zip file to your phone's internal storage, So that you can install Lineage OS 16 via recovery.

Turn the phone Off, boot into recovery again, Go to wipe, chose Advanced Wipe, And check : Dalvik Cache and Cache and system partition, Then swipe right, Go back to the main menu, Click on Install, Chose the Lineage OS 16 zip file from the internal storage, And swipe right (wait for the process to finish) , Now go back to internal storage, And flash the Open Gapps zip file.

That's it, Now check if the internet is working while using the moble data, If it does work, Then Congratulations ;)