I am not sure why Shopify hasn't sent an email to all POS customers, or added it to their status.shopify page, but if you turn WiFi OFF on the iPad you can process cash / alternative payment transactions. I would close the POS app before turning Wifi off, and then re-open it once you have made that change. Orders should re-sync after it has come back up.
Shopify drastically needs to fix this bug that prevents offline POS transactions, and improve their communication to end users as this is causing a lot of hardship. This should be emailed to all POS users by Shopify as we had to write down SKU's manually before realizing this fix.
I mean, it's par for the course...
After Shopify Unite 2018 we were told additional improvements and resources were planned for POS. There's been little to no change. Certainly nothing meaningful to the POS workflow and processes. The support teams remain minimally trained on POS.
POS isn't a large draw, I get that, but it's real disappointing to be told improvements are on the way and nothing happens.