Firmware v4.6.4 introduced a new Mediatek WiFi driver and some bugs with it, affecting mainly old AC devices.
In V4.7.0 the problems are not fixed yet.
To check if that is your case you have downgrade to v4.6.2 an check if WiFi it's OK.
There is a development test firmware provided by a moderator that runs fine too.
Looks like Mediatek is aware of the problem and many of Flint 2 owners, me included, are waiting for a fix in the next firmware. Meanwhile we are using v4.6.2 or de development version.