IOS update means no wifi

Last night I updated my IPhone 6S to 11.2.6 I can no longer access my home wifi network - system claims the password is wrong. I have

• turned on/off.
• I have reset router.
• have manually entered the (correct) password multiple times, and was very careful about the entry
• all other devices still connect
• "forgot" the network and then retried it

After one power on/off bout the phone found and accepted the network and password. I suddenly had wifi. Ten minutes later, there it was gone. And has not come back.