Need Help Cancelling iCloud Storage Plan Without an iPhone

Have you tried it like this?

How to cancel an iCloud+ subscription without an Apple device - Apple Support (CA)

If you no longer have your Apple device but you need to cancel your iCloud+ subscription, learn what to do. 

Cancel an iCloud+ subscription online 

1. Sign in to reportaproblem.apple.com with the email address or phone number and password that you used for your Apple Account. Forgot your Apple Account password?
2. Tap or click "I'd like to," then choose "Request a refund."
3. Choose the reason why you want a refund or to cancel, then choose Next.
4. Choose the most recent charge for your iCloud+ subscription, or the iCloud subscription at the top. Then choose Submit.

Refund decisions can take 24 to 48 hours.

“Need Help Cancelling iCloud Storage Plan Without an iPhone: Hi, I no longer own an iPhone but I’m still being charged for extra iCloud storage. When I log in to my Apple account online, it tells me to manage this subscription on my device — but I don’t have an iPhone anymore. Can someone please advise how I can cancel my iCloud storage plan and return to the free 5GB plan? I want to make sure I’m not billed further. Thank you.“

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iCloud Drive Expiration:

You will see the subscription in your Apple Account; it will then disappear at 12:00AM the following month in the day it was purchased. Log in here to see the date remaining, until the iCloud Drive Expires: reportaproblem.apple.com

IMPORTANT:

Export ALL of iCloud Drive: Be certain you back up you iCloud Drive prior to this expiration. Use my User Tip for that: Getting a Quick and Direct "iCloud Drive-to-External Hard Drive" Download - Apple Community