I cannot buy a new iPhone now therefore I'm still on iPhone7. iOS recently updated to 15.4.1.
I too access email through Mail being two Gmail accounts and one AOL account. I do not use an app for either Gmail or AOL.
I have fought a problem long before the recent update in that one of my Gmail accounts disappeared. The other Gmail account works as it always has and both were added at the same time some years ago. Both work without issue from my PC laptop connected to wi-fi hot spot as well as the AOL account. I have not added nor used any other email than the three mentioned.
When attempting to add the Gmail account that disappeared back via Mail I get this message : "Your browser has cookies disabled. Make sure cookies are enabled and try again."
On my iPhone 7 I use Safari browser and sometimes Firefox. I've used Safari since purchasing the iPhone and added the Firefox app later. I had no issue with the missing Gmail account but now unable to add it back. I've done as the message suggests numerous times to the point I give up, the same fix I have read on Apple website support making sure cookies are enabled on both Safari and Firefox browsers to no avail. I reset the IPhione truning off and back on, reset neetwork settings, all Ii can think to do and check the cookies enabled . Again, the other Gmail account works flawlessly as well as the AOL account having been set up via Mail. Suggestions? Incidentally , I've even enabled cookies on the Firefox browser on my PC thinking something bizarre there has caused the issue. Nope.