Just create a network with a different name in the app and disable 5 GHz band only for it, so that the created network will be only in 2.4 GHz.

I just bought Deco M5 and was quite dissapointed by its 5ghz performance. 2.4 Ghz band is disabled. On 5 GHz band: I usually get downlink ~200 mbps and uplink ~ 650 mbps (used speedtest and PassMark...