I have exactly the same problem. It started approximately when Opera upgraded to 53.0. I had made no changes to any settings in Opera or to the PC's internet connection when it started to happen.
If a site is unreachable in Opera, it can still be reached in Chrome, and pinging it gives low response times.
It is very intermittent. Some tabs work, others don't. Some sessions work fine, but next time half my tabs won't work. Also, if I try the same site again in Opera, it may work, or it may work if I close down Opera and restart it.
I'm running Win10 version 1803 with 8GB RAM, Opera version 53.0.2907.99.