HTC One M7: Remove InsertCoin ROM and Restore Stock Android 4.4 and Sense 6.

[Android Question]

Hey guys, I recently bought an HTC One M7 second hand from EBay. The phone was pre-flashed with InsertCoin (Update 4.0.4, KitKat 4.4.3)
I've been experiencing a number of issues since I update. Such as the permanently on R symbol and inability to use the mobile data.
I've been wondering how I can reflash the stock Android and Sense 6 into my phone.
Or maybe find a fix to the permanently on Roaming (But I'm not really on roaming) issue. I've looked all over the internet but haven't found a solution to the problem on roaming.
My phone is an unlocked, international version. I don't live in the US, so carriers and the SIM is likely not an issue.

 

[Hadron]

To return to completely stock you need to relock the bootloader and run a ROM Upgrade Utility (RUU). You can find a number of these here: AndroidRUU | The #1 source for Android RUU files or here: Shipped ROMs

The RUU does have to match the phone's original branding and region. If it's an unlocked international then the most recent HTC Europe RUU might be worth trying: RUU_M7_UL_K44_SENSE55_MR_HTC_Europe_4.19.401.9_R_Radio_4A.23.3263.28_10.38r.1157.04L_release_353069_signed_2-1.exe. That's not Sense 6, but if it runs then hopefully you'll be able to update over the air once that's done.

The other alternative would be to flash a different custom ROM. You can find some alternatives here: One Android Development - XDA Forums (I'm running ViperOne, for example).