Problem setting up second iCloud account

Hi I’ve been using Mac’s for years; my Apple ID probably originates before 2000. I have an old MacBook Pro running high sierra, an iPad and iPhone both running the latest iOS.

with BT closing its cloud service I’m looking for an alternative home for the files I store on their server. These are stored, not sync, and if required are downloaded to the MacBook Pro through a web browser.

I’ve found, from searching the web, that I can have more than one Apple ID, which seems to be the answer since I don’t want these files visible on my devices when signed in under my main Apple ID. The idea being to purchase storage under the second Apple ID, keep the devices signed into my main Apple ID and only access the second I’d when I need to access the documents stored through either a separate account on the MacBook or through a web browser.

I’m finding it impossible to setup the new account. Whatever I try, be it through the web, adding a new user account on the MacBook then going to the apps store, or going to the appleidApple web page they all fail.

The new account on the Mac got furthest - getting verification through a text message then an email to the id address. But when it came to signing in the account wasn’t recognised. Generally the message is that the account can’t be setup at this time.

Have I misunderstood the allowable number of Apple id’s and iCloud accounts. If not why am I having this problem. All the devices have only ever had my Apple ID so it’s not as if there is an unknown history stored in them.

your advice would be welcome.