Mac mini (Late 2012) Wi-Fi unpredictable

My Mac mini (Late 2012), using OS 10.10.5, connects to the Internet using a Zoom cable modem/router with wireless-N. Other computers and iPod Touch logon without trouble.
My computer sees the network, but only logs on sporadically.
Some days it connects me immediately upon startup.
Other days I can restart the computer and then it works. Sometimes I turn Wi-Fi off and restart and it might connect.
Sometimes, if I logon through my wife's account, it will join the network and sometimes it will not. It can take several tries of these various work-arounds to finally get connected and can take hours.

Is there a solution that will allow me to logon to Wi-Fi every time?

Should I buy an external device that will more consistantly communicate with my Zoom router?

Thanks!