Old AP One issues

I have 6 older AP Ones that I bought in 2012. In the last year, user’s have been complaining that they are connected to the AP with 3 or 4 bars, but can’t connect to the web. Usually a reboot helps, but it returns within a couple days. Are these units just too old to work well with newer phones? Users have been complaining, too, of poor speeds. These are running firmaware 3.5.3 build 1606.
How do I best diagnose performance issues?
Thanks in advance!

You may open a support ticket here, or you may want to load the 3.5.4 firmware see if that help.