Hi, Hjm. I've been away so haven't had the chance to reply, but another user below, Sne0108, has provided the exact answer that I would have, so I've simply copied it here. Also (maybe obvious), you need to uncheck the box that specifies to keep the SSID name the same for all networks, assuming as in my case that you want to be able to choose the 2.4 explicitly:
Log into your modem (192.168.2.1) and click on "manage my wi-fi". Then in the new popup window top right you'll see "Whole Home Wi-fi, Let Valérie optimize Wi-Fi throughout your home " click on the toggle to turn it off. after that you'll see 2.4 & 5.0 both show, and below that you'll see the button for "advanced settings". Now, you can set 2.4, 5.0 and guest separately with individual names, and also turn off the broadcast SSID (what i mainly wanted)