My cards are not showing up on my other device

Almost always this is an iCloud problem rather than a Reps problem. Reps has no account and no server of its own, so syncing happens entirely through your own iCloud account.

Work through these in order.

Both devices signed into the same Apple Account. Easy to get wrong on a family iPad or a machine you inherited.
iCloud Drive turned on on both devices, in Settings or System Settings.
Reps allowed to use iCloud Drive in the iCloud app list. This gets switched off during storage cleanups more often than you would expect.
iCloud storage not full. A full account stops accepting new data silently, which looks exactly like a sync bug.
Open Reps on both devices while connected. Sync happens when the app runs, so a device that has not been opened in a fortnight has nothing to catch up on yet.
Give it a minute. A large library with many photos and drawings takes longer than a handful of text cards.

Two things that are working as intended

Streaks and focus statistics do not sync. They are recorded per device, so your iPhone and your iPad will show different streak numbers. That is by design, not a fault.

Offline changes wait. Reps is fully functional with no signal, and anything you do offline uploads next time the device is online. Nothing is lost in the meantime.

Still stuck

Write to [email protected] with the devices involved and their OS versions.

More answers on the help page, or write to [email protected].