Connecting an iPad to a printer on another (LAN) network
For many years, I have been using Readdle's Printer Pro application for iOS. The app costs a few dollars, but seems to do the trick for me. It also works with all other Readdle apps, such as Scanner Pro, and Documents 5.

Printer Pro - print documents, photos, web pages and email attachments by Readdle
https://appsto.re/ca/hVxCx.i