If you didn’t fancy using the rails console workaround, another option is to enable 2FA for your admin account and then you use the authentication code to confirm granting admin rather than email confirmation (and you’d also have a more secure admin account as a bonus 
)
Though it is unusual that you’re not seeing a corresponding email log for it. 
Did you manage to track down the issue in the end?
I think there was another similar report recently too - Grant admin confirmation mail not sent