Update December 14, 2021
I received my mesh system yesterday from Amazon. TP-Link Deco S4 with 3 nodes. I was very satisfied with TP-Link brand products (range extenders, USB wifi external antenna for a pc, etc.) That and the cost ($110) were the reasons. I felt 400-500 dollar system would be overkill for my needs.
I downloaded the user guide and read reviews at Amazon yesterday. Since I wasn't sure what type of connection my gateway used, I called my ISP. Out of 6 choices my iphone deco app listed, it was 'Dynamic IP'. After installing the deco app on my iphone, I followed the instructions. The app showed the type was Dynamic IP. So, that was a relief. I set up the first node (satellite or unit) with ethernet cable to my gateway. it was easy. Deco app created the network. I followed one of the Amazon reviewers' recommendation, and set up the remaining deco units in the same room where my gateway and first node (which now works as a router) are located (second floor bedroom). No problem. Then, I unplugged those 2 units and placed them on first floor and in the basement.
I tested wifi speed. (speedtest by Ookla). Everywhere. Next to gateway, first floor, basement. I used both my old 7S iphone and a new HP Chromebook. Speed with T-Mobile's network was about 40-50 mbps. (Not much change. Considering the signal is coming from a cell phone tower.) Then, I switched to Deco's network. The speed went up to 120-130! Even in the basement. That was my whole purpose. Increasing the wifi speed down there, two floors down. If it stays like this, I will be a very happy camper.
Few thoughts:
1-Two networks. t-Mobile and Deco. Some say it may cause wireless interference and suggest I reconfigure my system.
2- Keep everything the way it is. Some devices, especially the ones that require 2.4 gHz connection are using T-Mobile's network. They are working fine at the moment.
3-Disable wifi on T-mobile gateway, and use Deco's router and its network? In this case, I need to reset every wireless device. Doable but sort of annoying.
4- Use Deco mesh system as AP. (access point) Apparently, some people have had issues when they used Deco as a router. This method disables some features on Deco. (port forwarding, guest network, time limits on kids, etc). I don't care about those.
I will wait few days or a week and see how the mesh is working.
I can't tell what method will work the best. I am open to any suggestion from network gurus here.