Many thanks Peter for your helpful reply. I suspect the issue here is that we have a custom theme that manages logins but I’d have to check that with the developer who tends to be a bit unresponsive. WooCommerce is not required, so not present. I’ve no idea what a modal window is, and there is no membership plugin present other than Restrict which is set up just to manage page access permissions.
I guess I’ll just abandon the idea of using 2FA until I can check with the developer and maybe find a way around the issue.