@dgwx it may help to do a full reset to factory defaults on the RT-AX58U. Many important fixes/changes were introduced in the first few firmware released for this model and it is best to do this now.
You should then change the control channels and bandwidth from Auto to manually selected ones. No special skills needed here. Choose 1, 6, or 11 for the control channel and 40MHz bandwidth for the 2.4GHz band and test all three, take notes, and choose the one that works the best for your network environment and client devices.
For the 5GHz band, choose 80MHz bandwidth and test as many channels as possible (that are not DFS channels). No rush to test all the control channels in one go, one hour or one day. This is up to you.
The notes you should be taking include not just a speedtest from your various client devices, but also your impressions of how responsive the 'net is when just surfing normally too. Maximum speeds mean little if the network feels slower doing it.
80MHz bandwidth is good for the 5GHz band because of freeing up the channels faster (to share with neighbors' AP's) and self-limits the coverage to the size of a normal home (as you have).
Also, look at the basic default settings suggested in the M&M Config guide (link in my signature below).
I'm a little confused about why the network needed to be forgotten if you used new SSID's?
The RT-N66U was turned off when testing the new router, correct?
Edit: for the SSID's, make sure they are 8 alphanumeric characters with no spaces, punctuation, special characters, or smiley faces in them. The passwords (for the SSID's and the router's main password) have them be the same with 16 character lengths max.
HTH.