Until we hear back from @solidus1983 if he has a stock build for the Flint v2 (GL-MT6000), I would prefer to not recommend flashing the ‘snapshot’ build that OpenWrt, the organisation, has for the GL-MT6000. Snapshots are really, really new attempts at building firmware for devices. To say they’re not even ‘beta’ quality is an understatement. They are developer attempts. You will have problems.
I’d prefer to hear what @solidus1983 has to say. He’s quite intimate with the inner working of the Slate Plus, Slate AX, Flint v1 if not the Flint v2.
Actually installing stock OpenWrt on a GL device is pretty easy; instructions are on the device page by OpenWrt. They refer to U-boot; U-boot usage is explained in the GL Documentation for Debricking.
What I would do is make a backup of your Flint v2’s current state as it stands before proceeding in any direction. Installing the OWRT snapshot requires some foundation skills but it’s not rocket science. If you can handle this HOW-TO, you’ll be far more able than most to flash vanilla/stock:
(The thing about all this is that you would have this exact same trouble if you got the ExpressVPN version so don’t feel bad; the Flint v2 will serve you better in the long run. You just jumped into ‘the deep end’ a little too early. It’s still better than being late. 
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