I think he meant Google was the easiest to remember that isn't the ISPs. I haven't used my ISPs DNS servers for about 10 years, no matter what ISP I've been with. I used OpenDNS for a few years but have used Google's DNS for the past 5-6 years and never ever had an issue. TalkTalk have had a number of issues with their DNS servers in the past couple years that I'm aware of, including a 24 hour period of very intermittent service (which was blamed on their DNS servers), and my connection ticked along fine.
I used to have OpenDNS memorized, but I would need to quickly Google it now. I think I'll remember Google's DNS till the day I die though. 8.8.8.8 and 8.8.4.4. As suggested try setting a custom DNS on the ZyXEL to see if that fixes it.