I've received an email saying that it's time to renew:

Your online number expires in 30 days
The online number: XXXXXXXX will expire in 30 days. To keep using this number, please sign in to your account to extend the subscription.
What do I need to do?
Sign in to your account. You can extend your number by 3 or 12 months. There won't be any break between subscription periods – the new one will pick up right where the old one left off.

But I've signed in and can't find any way to renew my subscription and online number. Do I need to make new purchases?

Any help would be appreciated.

The easiest way to renew is to cancel your # as it becomes due and repurchase it.
You won't lose the # as it is reserved for you for 90 days following expiry/cancellation.