router dropping devices/not allowing them to be added

Nighthawk x6- ac3200 tri-band router- its not a choice in the drop down menu

just downloaded latest firmware verion (after multiple failed attempts)

My problem: house size 4300 sq ft, single story, no inner brick walls. i can find the network out in the street, in the back yard and everywhere in the house.

the router is located on top of a bookcase about 10 ft off the ground in the very center of the house.

it has been awesome for 3 years that i have owned it.

recently i have been adding a lot of wifi connected devices to be run from a smartphone app, mainly lights, garage security camera/control and video doorbell-all of these devices are 2.4 gHz. These devices usually add pretty easily after 1-2 trys but recently some have been kicked off and i can't get them back on.

I am not a gamer, have 1 computer, a laptop, tablet, smartphone and printer on the network. No one else accesses it except the ocassional friend who visits. I am planning to add more devices and just want to solve this issue. Many of these devices are not active at the same time.

I have logged into the router but can't figure out who/what is assigned to the mac address and can't change their names if i could identify them.

it has been suggested i get an extender or switch to a mesh.

Please note I am NOT router savvy so any help has to be at the "see Spot run" level to be useful to me.

Any/help/suggestions would be appreciated.

also i have 2 questions that i can't seem to find the answers to so here goes:

1. Does a mesh alow for sevices to attach to the satellites and not crowd the bands on the router?

2. Does an extender add devices to the extender itself and not the router so the router is not crowded with devices?

What recommendations would you suggest to help me solve my problem?

thanx so much for wading thru this and offering suggestions to help me-