Hello,
How is the printer connected (USB, Ethernet, Wi-Fi)?
According to the Canon support pages, the latest printer driver appears to be for OS X El Capitan 10.11.
https://www.usa.canon.com/internet/portal/us/home/support/details/printers/support-inkjet-printer/mx-series/pixma-mx870?tab=drivers_downloads
https://www.canon.co.uk/support/consumer_products/products/fax__multifunctionals/inkjet/pixma_mx_series/pixma_mx870.html?type=drivers&language=en&os=os%20x%2010.11%20(el%20capitan)
An El Capitan driver may or may not work with macOS Mojave 10.14 as well. Do you know the source/version of the driver that you tested earlier?
You could perhaps try resetting the printing system (Reset the Mac printing system to solve a problem - Apple Support) and re-add the printer (Add a printer on Mac - Apple Support) with a fresh download.
If the above printer driver (US, or UK) does not work, (reset and) try the Canon MX870 series driver from the Canon Printer Drivers v3.4 for macOS package at https://support.apple.com/kb/DL1928?viewlocale=en_US&locale=en_US .
If nothing else works, you may even want to have a look at Gutenprint drivers (there are experimental MX870 drivers).
Install drivers from trustworthy sources only (such as Apple and the printer manufacturer).
Safely open apps on your Mac - Apple Support