Verizon FIOS G1100 router - 2.4 GHz radio weak signal in small business environment

An acquaintance of mine owns a small food spot; 15 seats. There are three dozen derelict wi-fi signals on the 2.4 Ghz spectrum. I enabled an advertised SSID on the 5 Ghz frequency, where there were only three derelict signals, and wi-fi speed increased from 0.20 mbps to 90 mbps down. Wired speed is 100 / 100 mbps.

The problem: Some employees and customers have older phones that can't connect to a 2 band WAP, therefore the 5 GHz spectrum is not available to them. None of the 3 clear channels in 2.4 Ghz range allow better connection/speed. How can I improve the performance? Only option I can think of is to increase the signal strength above 100%. Is that advisable? Any other recommendations?