@Xixin 

You have two separate WiFi networks in this house: one from TP-Link Archer, another from Deco mesh (two M4). It does not matter if you will use same or different SSID for them, iPhone will treat them as different networks.

Smartphone (any smartphone) will switch between different networks only when signal from the current network becomes weak.

In your case, you have two more or less simple options:

1. Turn off 2.4GHz in TP-Link Archer and in Deco mesh. WiFi 5GHz signal is weaker, and that will force your iPhone switch networks sooner;

or

2. Get yourself one more Deco unit, that could be M4 or M5. Add it to house Deco mesh, which will now become three Deco units mesh. Bring new Deco unit to where TP-Link Archer is and connect it to Archer with short Ethernet cable. Turn off WiFi in TP-Link Archer.

You now have all house covered my just one WiFi network, which is from Deco mesh. You iPhone should migrate to nearest Deco faster, because Deco mesh can communicate with iPhone and provide it with optimal Deco unit to connect to.

@Alexandre. understand, thanks for your help!