If your AirPort Extreme is a simultaneous dual band model......then it is not possible to turn off the 5 GHz band without also turning off the 2.4 GHz band as well.
If your AirPort Extreme is an older dual band version that did not support simultaneous broadcast of 5 GHz and 2.4 GHz, then the way to disable 5 GHz would be to enable the 2.4 GHz broadcast only.
So, the big question here would be.......what model of AirPort Extreme do you have? Look on the bottom of the device to locate the model number.....which might be embedded in the foam pad base. The model number starts with an "A" followed by 4 numbers. Post back with that info.
On a separate note, I'm not sure why your ISP would prohibit 5 GHz broadcasts, since just about every router sold on the planet in the last few years does broadcast a 5 GHz signal. You might want to get more clarification from your ISP.....or look for another ISP that is a member of the real world.