I opened a case with Apple, followed below steps which worked for me.
Installed 11.2 yesterday through phone (don’t upgate through phone).
Steps
1. Take a phone backup on computer through iTunes
2. Restore phone (iPhone 8 Plus) through iTunes In desktop. It was approximately 2.75GB
3. One done, setup the phone as a new iPhone. Don’t restore from backup at this point.
4. Once all the phone setup done, restore the backup taken in step 1.
5. All done... earlier my wifi speed was showing ~8mbps. Now it is showing ~40mbps
I hope this will help!!