A couple of months ago we had a requirement to extend campus Wi-Fi coverage to another building that is not physically networked.
It was situated further along from an existing Point-to-Point Ubiquti Airmax installation. The new building that needed to be connected had clear line of sight (LoS) of the existing base access point. So the simple solution was to convert the existing campus PtP install into a Airmax AC PtMP setup. To do this we required a replacement of the base stations antenna from Rocket Dish (RD-531-AC) to a Sector (AM-5AC21-60) that supports PtMP.

The steps I took are below:

1. Bill of materials:
1 x Ubiquiti Sector Antenna (AM-5AC21-60) 60 degree beamwidth model.
1 x Ubiquti Rocket Prism 5AC Gen2 Access Point.
POE switch for station end.
Consumables (cables, injector etc).

2. Backup to config of the current base station, and export it to file. From Air OS 8 Web UI > Settings > Back up Configuration.
3. Restore config onto new building base station and change mode to Station PtMP. Air OS 8 Web UI > Wireless > Wireless Mode > Station PtMP, Auto Channel Width ON, Antenna RD-531-AC-31dBi.
4. On the existing client station (for the first building), change settings as above to Station PtMP.
5. Once gear is installed – including sector antenna installed, existing rocket dish moved to second building station. Configure the base AP wireless mode. Air OS 8 Web UI > Wireless > Wireless Mode > Access Point PtMP airMAX AC, Antenna AM-5AC21-60-21dBi. As all our stations and antenna are airMAX AC devices use this mode in PtMP configuration.

When a Rocket Prism AC Gen 2 is in PtMP mode, channel bandwidth is split. As the max is 80mhz channel, what this mode does is split it based on how many stations you have. In this configuration we have 2 stations so each get 40mhz. This is why under Air OS 8 Web UI > Wireless > Channel width only shows up to 40 MHz. In this mode I was able to see bandwidth capacity of 338mbit per client station. If you times that number by two its essentially the max bandwidth the Prism model can do in PtP AC mode (650mbit).

So to recap when planning out a Ubquiti PtMP design remember the following:
Work out the required line of sight, antenna beamwidth and gain (Sector vs Rocket Dish).
Only use Access Point AirMAX AC PtMP mode if all your gear is Ubiquti AirMax AC, If not use Access Point PtMP airMAX mixed.