Hello,
I have a lot of experience using openwrt , captive portal and I can tell you this project is a very long shot. I must find right openwrt version for all 4 devices . openwrt version depends by hardware version . Ex. for wa701nd are versions 1.0 2.1 , 2.2 etc. Some openwrt versions doesnt come with a captive portal so I must add those packages and final firmware must be fit in 4mb flash memory. tplink wa701nd, wa801nd, wa901nd and wr7210n come with just 4Mb flash.
Also I think you need a radius server and an web admin to create vouchers.
Maybe are other solutions . Devices are an same network or you have more locations, 1 device in each location. if are in same network we can use mikrotik.
I think toy are aware about hotspot -captive portal issues:
- user must open a browser , access a http (not https ) page and will be redirected to landing page for authentication with voucher.
- we can use https redrect but if default webpage for browser is google will not work.
- some smartphones got a notification ( ..open a broweser) but not all - depends by android/ios version/settings
please don't assign me this project . we must talk first.
Regards , Octavian