I can’t see a way to directly specify the frequency on the phone. (At least, not in the GUI.) Do you have your Wi-Fi configured so that it uses a separate SSID / network name for each frequency band?
My Wi-Fi is configured so that the same SSID is used for both 5GHz and 2.4GHz. This works well for my laptop. The Librem 5 does have worse 5GHz reception than my laptop, but the main issue I get is that it takes too long to switch to 2.4GHz when the 5GHz signal strength has decreased, such as when I walk into the living room. I usually find it quicker to go into the settings and manually disconnect and reconnect to the Wi-Fi network, rather than wait for it to go over to 2.4GHz of its own accord. (At least, I presume this is due to the 5GHz signal strength. I never actually checked which frequency it was using.)
Oddly, it is currently sitting about 60cm from the Wi-Fi AP, which is where it was when I last rebooted it, and it’s telling me it’s put itself onto 2.4GHz, even though it should have perfect 5GHz signal here. This is actually great, because it means it won’t have issues switching from 5GHz to 2.4GHz, but I’ve no idea why it’s done it. 