Sometimes we forget that for the communication to be reliable, signal must flow both ways. Clients will happily connect to far away APs, but won't be reliably heard back.
Solution is to use more APs and lower their radio tx power. A lot of customers push back on the approach because they have only one gaming router monstrosity in their country home and it works perfectly, not understanding that their downtown office with 150 devices have nothing in common.
DECT is a much, MUCH more old, simple and robust protocol. Modern signal processing tech can get you insanely far there.
You have a bunch base stations forming a logical channel with a single uplink frequency, and either a single downlink frequency on which every base station is precisely synchronised in frequency and time (simulcast), or multiple downlink frequencies (a multicast system).
A comparator selects, or votes, the highest quality signal from the base receivers based on either lowest noise or lowest BER, and then repeats that through all of the base transmitters simultaneously.
At the same time, in a multicast system, the mobile scans for and selects the base transmitter with either the highest RSSI or lowest BER.