Also, I've found the leaked MT7615 driver on github. I'm not posting here because I don't know if it is allowed. How can I compile this driver for my kernel to OpenWrt?
From a legal perspective, unless you obtained those blobs with your router's firmware, you are likely not permitted to use them. If you have the OEM firmware for your device, it should come with the appropriate blobs and licensing.
As this forum prohibits illegal activity, demonstrating that you have the legal ability to use any such blobs would be a good first step.
There are threads on this forum around MT wireless drivers that, to the best of my knowledge, are not encumbered with the same licensing requirements as many OEM proprietary blobs.
Ignoring the legal aspect for a minute (which isn't really an option), but from the purely technical aspect of it - kernel modules must match the running kernel exactly (version, patches, configuration), with very little leeway for variations, if you'd opt to ignore the mismatch. That makes it technically very "challenging" (read that as close to impossible) to get (keep) precompiled/ binary kernel modules working.
In fact, that blob on the link was provided to me by the manufacturer of the hardware. The firmware OEM provided was using OpenWrt Barrier Breaker and I'm trying to make it to work with OpenWrt 18.06.
I didn't post the link to the mt7615 github because of that.