[ONE Q&A, Help & Troubleshooting] Fluvance:
Tl;dr: OPO refuses to connect to my default home network, instead chooses roaming. Cannot text or call and data will not work on roaming (they used to).

BACKSTORY

Hello, my OPO was working almost perfectly for 2 years without any significant damage like water or drops. Then, in November of 2016, I went on a trip to Europe for a few months (I live in Canada), and used my OPO occasionally there, by roaming when I needed to make calls. (Used my regular cell plan with an extra worldwide option)

When I got back to Canada, I had a stopover flight in Toronto, where I was able to connect to my HOME network and make a call and some texts. Also used some data during this time. Then, I flew the rest of the way to Vancouver. There, I was able to connect to my HOME network and send some texts and use data for about an hour. That was at the end of February.

THE PROBLEM

Since then (last 1.5 months) I have been unable to connect to my HOME network. My OPO will only connect to the AWAY/ROAMING network, even when I go into Network Operators and manually force HOME - it just says it's unable to connect at this time.

When connected to AWAY, I typically can not do anything. However, sometimes, I will be able to receive calls, and send and receive texts. No data, and no sending calls. Before this whole problem started, I was usually able to call and text and use data on AWAY.

WHAT I HAVE TRIED

Choosing different Network Operators, different Network Types (WIND is WCDMA). Problem persists.

Deleting and creating new APNs from the current information on carrier's site. Problem persists.

Ensure that my OPO can read the IMEI. It does

Went to carrier (WIND, aka Freedom Mobile) and tried the store's demo SIM that uses an alternate account. Problem persists.

Carrier tried my SIM in their own phone. Worked on that phone.

Carrier gave me a new SIM for my account.Problem persists.

Reset my OPO to factory defaults, including apps and data. Problem persists.

Format my OPO and flashed different ROMs (including various CM and stock releases from KitKat to Nougat). Problem persists.

Flashed different modems available for OPO from the various original CM11 ROMS, as well as one from a CM12 ROM. Problem persists.

Tried the SIM card from a friend's phone on a different carrier (Virgin Mobile in Canada).Problem persists.

Removed my StyleSwap cover and ensured antennas and antenna pins were clean and making contact. Problem persists.

Tested Wi-Fi, Bluetooth, NFC, and GPS to ensure other antennas were working. They are functioning

Use online tool to see if IMEI is blacklisted. It is not, and problem persists.

Took my OPO with me when travelling to various parts of British Columbia over 100KM away to test different cell masts. Problem persists.

CONCLUSION

That is as much detail as I can think of. I would be happy to provide more if it may help. I already had a chat with OnePlus's live support and email, and was told I would need a repair, which would cost a few hundred dollars.

I am also thinking about buying a used/broken OPO for cheap and swap the motherboard, however there are not many available near me right now, and this also carries some risks akin to buying stolen phones.

I am pretty tech savvy, so feel free to go into as much detail as you can.

Thank you for any and all the help you are able to give!