Deco M9 Plus V2 network refuses to connect devices to nearest mesh unit
I've got a mesh network consisting of two Deco M9 Plusses and two M4Rs. Both are V2.
I've got one long-standing issue: per the title, the network either refuses to connect devices to the closest mesh unit, or it takes a ridiculously long time to do so. Yes, fast roaming and beamforming are enabled. Yes, my WiFi adapters support fast roaming. My laptop's adapter, in question, is an Intel Wireless-AC 9560. I've tuned it to maximize compatibility and performance; the 2.4GHz and 5Ghz bands are restricted to 20MHz only to reduce interference, fast roaming is maxed out at highest activity, and transmit power is set to the highest possible setting as well.
Right now, I'm sitting one feet away from an M9 unit placed upstairs; my phone is still connected to the primary M9 unit serving as the "router".
Is there any way I can rectify this issue, or is this a limitation with the Decos? The same thing happens when I position my devices next to one of my M4Rs; I often end up connecting directly to the primary Deco M9. This results in nonnegligible packet loss and jitter, so I really hope it can be fixed.
Also, just to double-check: does the M9 Plus V2 still have no web-based interface with firmware v1.5.1?