If you see the screen shot, from CiviCRM 4.7.22 on Wordpress, isn't there supposed to be an "Allow Other Amounts?" option to check under the "Recurring Donations" option? According to these instructions, there is supposed to be.
Check the Allow Other Amounts? box to allow donors to enter custom amounts for their donation. This will add a field to your contribution page labeled Other Amount. Selecting this option will allow you to set a minimum and maximum amount that can be donated, if desired.