What firmware are you using and have you gone into LuCi and configured both radios to use the correct country and the same channels as your Flint 1?
Depending on your WiFi settings that could be caused by the routers mt76 WiFi driver, since Apple devices have been a problem for the driver in the past.
https://forum.gl-inet.com/t/flint-2-poor-wifi-speed/36133/13
https://forum.gl-inet.com/t/gl-mt6000-poor-wifi-coverage/35915/16
Is the latency only an issue from your one phone or all devices that use 2.4GHz? And what country do you have your radios configured to use?
I’m asking the questions since I don’t currently experience any latency issues, but I could temporarily change a few of my settings to match yours in an attempt to replicate your issue.