Some websites won't open only when using wifi

I just got my home internet yesterday and I had a heck of a time getting everything to connect right and work. This device barely qualifies as a router.
The current problem I have is that on my two tablets and phone, some of the websites I try to go to won't open. If I turn off the phone's wifi and use mobile data, all of the websites open fine. My computer that's wired to the router finds all the sites with no problem. When I unplug the cable and switch the computer to wifi, it can't find the same sites that the wifi devices can't find. I've restarted things over and over and created new networks that are 2.4 only or 5Ghz only and with WKP/WKP2 rather than the WPK3 option. I even unplugged and restarted the modem/router in case rebooting wasn't enough. What could cause some websites not to open only when using a wifi connection to the router?