It sounds like your current router is either over heating, or on its way out.
in my personal opinion, WI-mesh would be a waste of money, you can accomplish total coverage with normal AP infrastructure mode.
We don't give shopping recommendations here at superuser, there is a whole other community for that. so choosing new routers will be at your discretion.
This is how you would connect regular routers to essentially act as a mesh network.
(and save yourself a decent amount of money, WI-mesh is costly.)
The configuration would look like this:
Ubiquiti 4-port wired router
MODE = Router/Firewall
LAN IP address = "192.168.1.1 255.255.255.0"
DHCP-Pool = "192.168.1.2 - 192.168.1.251"
Asus AC router #1
MODE = AP
2.4 GHZ SSID = "Home-net"
5.8GHZ SSID = "Home-net-5ghz"
LAN IP address = "192.168.1.254 255.255.255.0"
DHCP = "disabled on this router!"
WPA2-PSK 2.4ghz = "MyPass1234!"
WPA2-PSK 5.8ghz = "MyPass4321!"
security type = "AES only"
NAT & Firewall = OFF!
WAN = disabled, with up-link connected via LAN port.
2.4GHZ Channel - 1
5.8GHZ Channel - 100
Asus AC router #2
MODE = AP
2.4 GHZ SSID = "Home-net"
5.8GHZ SSID = "Home-net-5ghz"
LAN IP address = "192.168.1.253 255.255.255.0"
DHCP = "disabled on this router!"
WPA2-PSK 2.4ghz = "MyPass1234!"
WPA2-PSK 5.8ghz = "MyPass4321!"
security type = "AES only"
NAT & Firewall = OFF!
WAN = disabled, with up-link connected via LAN port.
2.4GHZ Channel - 6
5.8GHZ Channel - 112
Asus AC router #3
MODE = AP
2.4 GHZ SSID = "Home-net"
5.8GHZ SSID = "Home-net-5ghz"
LAN IP address = "192.168.1.252 255.255.255.0"
DHCP = "disabled on this router!"
WPA2-PSK 2.4ghz = "MyPass1234!"
WPA2-PSK 5.8ghz = "MyPass4321!"
security type = "AES only"
NAT & Firewall = OFF!
WAN = disabled, with up-link connected via LAN port.
2.4GHZ Channel - 11
5.8GHZ Channel - 124
I would personally put a new router on the 1st floor and one on the third. check the coverage area before you buy three new routers, you may find you have decent coverage with two. Additionally, you had the right idea. Disable any and all extra features on the access points. You want your main router doing all the processing and hard work, the more you can unload from the AP's the better they will perform.
WiFi is half-duplex, which means that on any channel, only one device can talk at a time. If two devices try to talk at the same time, they would interrupt each other. WiFi is more like a one-lane highway; traffic can only flow in one direction at a time.
Since WiFi is half-duplex, only one WiFi device can transmit on a channel at a time. The more WiFi devices we add to a channel, the more we reduce opportunities for each device to talk. This is known as co-channel interference. If you would like a more detailed explanation, I wrote one for someone else here.
I hope this helps you in building a better network!!
Best Answer
Before you go buying new equipment check the air waves around you. If you are on the wrong channel the frequency you are using just might be saturated. Channels 1, 6, and 11 have the least amount of over frequency overlap, so choose one of them if one is open and available.