How can I restore iMessages in my iPhone?

Go to:

Settings > Your Name, iCloud, Media & Purchases

Tap iCloud

Are Messages turned on (green)?

If yes, when you deleted the messages they were deleted and you cannot retrieve them

If Messages are not turned on AND IF you have a backup you made from before you deleted the messages, you can restore your phone from that backup. BUT your phone will be restored to the date the backup was made, meaning messages sent or received as well as other data will be gone.

There is no real, practical way to restore messages; Deleted messages are gone.

You can restore messages from a backup but that is at the expense of anything newer since your last backup. See instructions here: https://support.apple.com/en-us/HT204184