Replacement remotes won’t work

First of all, we don't know what models of remotes you have been buying, this is a true Roku remote for Roku tv's:

Amazon.com: Roku TV Remote Control (Official Manufacturer Product) - Simple Setup, & Pre-Set App Shortcuts - Replacement Remote Compatible with RokuTV Models ONLY (Not Roku Players)

Roku devices and Roku tv's use different IR code sets than each other, so remote made for only for Roku device is useless on Roku tv. And only Roku wifi remotes have pairing buttons or use the two-button combination of Back & Home. 

Another thing your tv may not accept IR remotes anymore, might be defective? The cell phone app doesn't use IR nor Wifi, it uses IP the internet. So, defective Roku tv that doesn't accept IR remotes can still be controlled by app.