Unfortunately people experience issues if they upgrade to 7.6.4 even if they downgrade again.. that firmware seems to make some major permanent changes to the router..
Have a try though.. go back to 7.6.1 unless you have a very early Gen4 in which case 7.5.2 will work. Do a factory reset and see how it goes.
Set different wireless names.. short, no spaces and pure alphanumeric. Definitely a different name for 5ghz.
Set manual channels, 11, 6, 1 for 2.4ghz in that order.
Set around 36-40 and around 150 for 5ghz and see how that goes.