If band switching worked, good chance there's a lot of other wifi access points (or radio noise) in your area. The technician found a cleaner channel.
Doesn't last long because if every other access point was on auto, then something would eventually migrate to the "clean" channel you were on.
Can use a wifi analyzer app to visualize the wifi networks nearby and the channels they are using. Could help in moving your own router to a relatively cleaner frequency, but as always ymmv.
Also wifi speed is half-duplex, and divided across all devices on your network. "AC1300Mbps" sounds fast in theory, but when you have 10 devices that's really just 130Mbps each na lang, and divided between up and down so it could be 100/30, or 65/65.
Since wifi hops across each of your devices in turn, having one slow device on the network can considerably slow it down for everything else.
It's completely unlike ethernet which is full-duplex meaning same speed up and down, and guarantees the same link speed for every device.