Go to your Site Preferences >> Buyer management>> manage who can buy from you >> Edit
On that page scroll down a little to see this:
Change the number there.
Or scroll to the bottom and add that buyer's ID to ::
Manage exempt buyer list
No matter your block settings, buyers on this list can shop your listing. Use the exempt buyer list to let specific buyers through, regardless of your settings.Manage list
If the above is not possible on the app or mobile browser, switch if you can to a PC / laptop and use Chrome, to manage this.