Is this a Gen 1 or a Gen 2 Timberline 850? If a Gen 1, it most certainly needs a replacement controller as Traeger themselves state so on the WiFIRE Status Page ...
Current Firmware and Configuration Versions
...
If you have an AC controller running 02 firmware and are having issues connecting it to WiFIRE®, please call our Customer Service team at 1(800) Traeger and we will get you a replacement controller with the most up to date firmware.
EDIT: two similar threads going on at the same time ...you've already confirmed you're running a Gen1 Model ... point them back to the WiFIRE Status Page (
https://www.traegergrills.com/status) - that is where the above information is posted ...
If this is a Gen 2, Firmware Version 02.00.00 goes back to January 2020, and was quickly replaced by version 02.00.01 ... why? because it broke Wi-Fi Connectivity for a subset of WiFIRE Controllers ... and those impacted were NEVER able to get it working again ... been down this road way too many times to count in 2020 via assisting at least 40-50+ folks via the various Traeger-related FB Groups ...
With regards to Traeger's point on the Router / WAP performing "steering" of devices to the appropriate radio (2.4GHz or 5GHz), the easiest way to eliminate that issue is to temporarily disable the 5GHz radio, restart the Router / WAP, restart the App Device, restart the WiFIRE Controller and start from the beginning ... if it fails with ONLY the 2.4GHz radio "active" via the Router / WAP, then you've proven that the issue is tied back to the Firmware ...