@emhy 

It is possible your DECO5 tries connecting wirelessly to Main Deco, and because of weak signal between them appears disconnected. If you bring DECO5 to the basement where Main Deco is, will DECO5 stay connected and healthy?

Yes, it will stay connected if DECO5 close to the Basement Main Deco. (I think you answer my questions because the DECO5 is too far from the main deco, Thanks)

I have another two questions:

1. I wired the Main DECO1 to DECO2, 3 & 4 respectively. I expect the speeds are about the same. When I do a speed test near Main Deco, it is double or triple than the other DECOs. Any idea why?

2. I am using the Bell Fibe 1.5 internet. Can i do an access point set up? (see below). 

ISP modem (Basement) wires to DECO1 (Basement), DECO2 (1/F), DECO3(2/F) & DECO4(3/F) separately.

If i set up this, Is it true i can just use Bell's network? and I don't need to set up a new network name. Will each DECOs extends Bell's WIFI?

What is Pros and Cons for this?

Thanks for your help!!!

  @emhy 

There is a known issue with Deco, where wireless Satellite Deco strongly prefers to connect to Main Deco. TP-Link started to release firmware with ability for user to overwrite that, but there is no such firmware for Deco X60 yet.

1. I wired the Main DECO1 to DECO2, 3 & 4 respectively. I expect the speeds are about the same. When I do a speed test near Main Deco, it is double or triple than the other DECOs. Any idea why?

The speed should be the same. Check if your Satellite Deco do use Ethernet link to Main Deco, and not WiFi. If they use WiFi even if Ethernet wired, see this document: General questions about Ethernet Backhaul feature on your Deco

2. I am using the Bell Fibe 1.5 internet. Can i do an access point set up? (see below). 

You can, as long as Bell provided you with the router/gateway. See How to set up Deco to work in Access Point mode

If i set up this, Is it true i can just use Bell's network? and I don't need to set up a new network name. Will each DECOs extends Bell's WIFI?

In Access Point mode, Deco will keep providing its own WiFi under SSID/password you configured in Deco app. Deco mesh will not extend Bell WiFi signal.

What is Pros and Cons for this?

This document may help: What’s the difference between Access Point mode and Router mode on the Deco?