Print an NFC sticker with the URL https://simplcrm.co/tap/<gymSlug> — that exact host, because it is the one the app registers for deep links. A tap checks the member in straight from their phone browser; no app needed. If they have the app installed it opens there instead.
The member must be signed in at simplcrm.co/member on that phone — the tap sends them to sign in once, then straight back to the check-in.