In the terminal, you ping a known URL like this to test if DNS is working:
ping -c3 plugintolinux.ca
If that works, it's the website and not the dns servers you're using. If it doesn't, do this next:
ping -c3 8.8.8.8
That's one of Google's dns servers. If it works, then the problem is with a DNS server you're using and not your overall connection to the internet. It means that address translation (what DNS servers do) isn't working. If it doesn't work, then your internet connection is entirely down.
Now, since Ooma isn't working, too, that would suggest it is a connection problem to the internet because it probably uses different DNS servers unless you have DNS set up in your router and you're using it to resolve addresses for both. In which case, they would both be down if the problem was with a DNS server. However, I think DNS servers are just used in TCP/IP protocol and Ooma may use UDP for communication in which case I think it just goes straight to IP addresses.
If the problem is with a DNS server, reconfigure your network settings using an icon that is probably in your lower right in Cinnamon, MATE or Plasma. DNS servers are under the IP4 tab. Seperate DNS servers with a comma. I recommend trying OpenDNS at
https://opendns.org. The addresses to use are the bottom of the main page. As a side effect you may find that their DNS servers are a lot faster than your ISPs resulting in faster loading times.