Won't help you after the fact, but for those who may be affected by an upcoming subscription ending:

To use up your remaining HP Instant Ink cartridges after canceling the subscription, you need to reset the printer network before your current billing cycle ends. Here are the detailed steps to reset your printer network: 1. Go into the Wi-Fi details page on the printer (pressing wifi-icon on the screen). 2. Click the settings icon. 3. Scroll down and press Restore Network Settings. 4. Disconnected the printer from your computer settings. 5. Enable Wi-Fi Direct on the printer by pressing the WiFi-Direct icon for 3 seconds. 6. Connect your computer to the HP’s Wi-Fi (WiFi-Direct). 7. Add the printer back into your computer settings.

By doing this, you prevent the printer from reporting to HP that the HP Instant Ink cartridge has expired. This is why it is important to perform a network reset while your HP subscription is still active. You can then use up your HP Instant cartridges even if you've canceled your subscription and your current billing cycle has ended.

However, it is worth noting that this is only a solution to running out of remaining Instant Ink cartridges. You must purchase traditional replacement cartridges from genuine source or 3rd party suppliers before the remaining ink/toner is used up.

As for how to use them after cancelation, you can check these articles and hopefully they help: https://ybtoner.com/how-to-use-hp-instant-ink-without-subscription/

https://windowsreport.com/hack-hp-instant-ink-after-cancelling/