I'm not sure what the issue is. Most modern routers use both 2.4Ghz and 5Ghz as their WiFi frequencies. (6Ghz is a new frequency on WiFi 7 - a new standard) your older tablets and phone should be capable of talking WiFi 5 or 6 on 2.4Ghz. 2.4 is the "old" frequency but still in use by a wide variety of "legacy", oldish and new devices.
If you are having problems connecting on 2.4Ghz it may be a problem with your router and it might need to be replaced.
Take your tablets and phones down to the local mall that offers free WiFi, if they connect and talk you have a problem with your router.
What is your router?