Yes that does sound like old technology, but if it's only for the purpose of connecting a phone to, it should be up to the job
Depends where you are, I'm in London with 20+ 2.4Ghz SSIDs. If I connect to the 2.4Ghz network all my iPhones will switch over to 4G data due to WiFi Assist, as the 2.4Ghz loads data so slowly. If you disable WiFi Assist, you struggle to even browse to a web-page (well it takes upwards of 30 seconds). I confirm this behaviour at 3 properties in London (as I've moved), all equally dense in local 2.4Ghz networks, on virginmedia, BT and Sky.
Here is a video of how 2.4Ghz performs in my home in London for most of the evening:
In London you often have 4 flats on-top of one another, and then 4 either side, and across the road the same thing.
As we keep adding in a bunch of 2.4ghz gear (e.g. bluetooth earphones for iphones 7s and 8s), wireless doorbells, baby monitors, cordless phones, microwaves, wifi APs, bluetooth speakers, bluetooth keyboards and mice etc, the spectrum is becoming very undesirable, even for casual browsing.
My advice stands, install 5Ghz APs in this day and age, and certainly where you add APs ensure you are aligning standards, roaming between an AC device on 5Ghz down to a 2.4Ghz device at wireless G 54Mbps is not nice. When I used to work with Aruba Networking it was a huge no-no to roll out in this way.