I changed the restaurant's phone. What do I have to redo?

Three things, all under Settings → WhatsApp. Link the new phone: press the link button, open WhatsApp on the new phone, go to Linked devices and scan the QR, then wait for the badge to say Connected. Check the two groups: the managers group and the riders group must be groups the new phone is in, so re-pick them under Order notification groups and press Save groups. Check the Orders hand-off number, which is where a customer's own order summary lands after checkout; if it was the old number, change it. Then place a small test order and look at Message history for a row marked sent. The WhatsApp tab in the docs has the linking guide.