Netgear Nighthawk M6 Pro - Need to add ext Antenna

[DTGMI]

We are telecom/data dealers and have a 15,000 sq ft masonry building with steel roof.

We just added a AT&T Mobile Broadband service to our building - 4G - LTE - 5G

We will use it for WIFI for our employees to use for personal use (not connected to our corp network)

We just set it up and the signal strength is low to average and the AT&T tower isn't too far away - 1/2 mile

We are thinking of running about 20 ft of cable (M6 modem uses (2) TS-9 Ext port antennas) up to the roof top where we have a 5 foot high location to connect an external antenna to.

I've looked at various antenna - Wilson etc - and was wondering if any techs can point me in the best direction.

I can get the cable cheap but want the best antenna ... just adding a single one for now.

Thanks!