This thread will be of some assistance: viewtopic.php?f=7&t=70184
Now, that being said, some routers allow you to boost their wi-fi broadcasting power in the router settings. This is to increase signal strength and has nothing to do with how many people are connected to it, though. It's usually represented as DBM or SNR if your router supports it. I can't find an actual PDF manual for your device, (stupid Zyxel...) but the info page on the site for it looks to have directions on how to find what I'm referring to.https://qwest.centurylink.com/interneth ... radio.html
Go there, and tell me what options are on the wireless power level and 802.11 mode drop-down menus, as well as what they're currently set to.