If your ISP’s DNS goes down, your internet "stops working" even if the signal is fine. Third-party servers act as a reliable backup.
Some users report that after a firmware update, the ZTE MC888 Pro occasionally ignores manual DNS entries (a known bug on early firmware versions, e.g., V1.0.0B05). If your router stubbornly reverts to ISP DNS, you can configure DNS directly on individual devices while waiting for a firmware patch. DNS Settings on ZTE MC888 Pro 5G