Heyyo, by chance have you tried holding down the power button and volume down button? If you're lucky it'll go into bootloader mode and you can flash it via a PC using fastboot commands... If not? It will definitely need to be sent in for warranty repairs. Such is the way with any device be it $1000...
True it is a real alert, but it should be all the same test or real... So Koodo mobile users should definitely be receiving all alerts... Maybe OnePlus 3T doesn't have the necessary updates/support on OxygenOS??? I'd contact OnePlus and find out from them too as it could be a OnePlus issue.
For the record it is a test build of LineageOS 15.1 on my LeEco Le Max 2. I am one of the community developers for support for this device. LineageOS 15.1 (Android 8.1 Oreo). There is also official LineageOS 15.1 x2 updates out but I'm always testing my future updates.
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