You can refer to the server settings given below and check if you are using the same settings on your iPhone:
Incoming Mail (IMAP) Server
• Server: export.imap.aol.com
• Port Number: 993
• Requires SSL: Yes
Outgoing Mail (SMTP) Server
• Server: smtp.aol.com
• Port Numbber: 465
• Requires SSL: Yes
• Requires authentication: Yes
Your email login info
• Email address: Your full AOL Mail address (including “@aol.com”)
• Password: Your AOL Mail account password
• Requires authentication: Yes

Ref:- https://bit.ly/2JiCxZQ

Chicago, Computer, How to resolve imap.aol.com is not responding iPhone?