@ARGamingPK1 I'm not TP Link staff, buf I think the reason is Deco with weaker signal still has strong enough signal and Your phone "decides" not to switch yet, coz switching is expensive and may cause some lag in connection.

-40 dBm is near Deco and

-55 dBm is not near, but still close

I think it should switch somewhere near -60 or -65 but not sure.

Or You should give it a time and it may auto switch still.

Or try to turn off/on WiFi, hopefully it will connect to Deco near You.

And don't try fast roaming, it's not about better switching threshold, but smoother switching. I read somewhere on this forum, that fast roaming may be unstable on some devices and may cause slower Internet speeds, so better keep it turned off.