The only options left are on the base station side: more access points, repeaters or some type of add-on signal boosting antenna at the AP. All which Technochic has pointed out.

The best solution is to increase the APs or add repeaters. Using antennas to boost signal strength for transmit and receive is tricky and subject to many potential issues. It works OK in small locations. YMMV

If this is a large location, a professional signal survey of the facility can determine the appropriate number of APs and their optimal location to avoid creating blind spots in coverage.

While MIMO works to give better range and throughput with other MIMO components, it does not help with standard G (or B) units.