Satelite is not adding or staying active

Thanks for your reply. See my response below:

What Orbi model system do you have? There are many Orbi 6 models...(The model is have RBR750 for Router and RBS350 for satellite)

What happens if you swap RBS locations with each other?  (I did have the satellites in different floors after it was setup. It worked fine until about 6months ago. The speed shows up at 150mbps or sometimes 250mbps. Before it was stable at 455mbps).

What WiFi devices do you have connected? Brand and model... (I have 2 separate SSIDs created. I have smart device tied to the 2nd SSD, such as Roku sticks, garage door opener. Other devices are hardwired so I would think wifi should be much more stable and should provide more speed. FYI: i have never seen wifi speed upstairs over 400mbps. The router had to be installed at the basement since the builders will not move it as per their policy).

What Firmware version is currently loaded?

For router: version is V7.2.6.31

Satellite 1: version is V4.6.14.3

Satellite 2: version : V7.2.6.31

I tried to manually upgrade the firmware on one of the satellite but it will not work. It keeps saying invalid file. Another steps where I spent few hours.

What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too? (The modem is from Quantum Fiber/Centurylink. Their technician was here and tested the modem and told me modem is working fine. I tested it by connecting directly to the modem. I was able to get stable speed (it varied between 550mbps and 750mbps).

Be sure your using a good quality LAN cable between the modem and router. CAT6 is recommended. (All the cables I have are cat6 and cat6e. I changed those months ago. Like I said, it was working until about 6months ago. There was a power outage since then the satellites have been acting up.)

What is the size of your home? Sq Ft? The house is about 4400sq feet. it's a modified 2 story home (it has 3 levels).

What is the distance between the router and satellite(s)? (The router is in the basement since modem is there. The 1st satellite is in the main level where the kitchen is but is away from appliances. 2nd satellite is in the Loft. The devices are more than 30 feet apart for fact).

Does this happen with the RBS turned OFF?  (I did leave 1 of the satellite powered off. The other satellite did work but speed remained under 250mbps.
Does this happen with both wireless and wired devices? (at times its' mostly wireless. sometimes it is on wired. I did connect my laptop to the satellite and max speed i got was like 250mbps).
What channel configurations are currently set on the router? (The first SSID is set to 02 channel at 2.4ghz and 5Ghz is set to 40. I had it set to AUTO before but wanted to test it by changing it).

When you factory reset the RBS, did you reset the RBR as well? Yes. I did a factory reset on all 3 devices (Router and the satellites). Wifi speed varies and I am aware of that but I cannot figure out why one of the satellite continues to fail. It will either show up as Flashing white or Blue or Magenta then will go lightless. I find it quite frustrating as I have tried multiple options by reading the threads here or somewhere else. )