Error messages and more

Every time I visit certain website i.e google, Facebook. I get an error message. It happens to a few other sites. Also when my iphone is connected to my wifi Instagram snapchat and iMessage won't work. I've tried changing the DNS server, resetting and reconfiguring my router and deleting the cache on my iMac. This problem doesn't happen on my windows computer. If someone has any ideas on how to fix this please let me know.