When you go into a restaurant or store and the vendor has failed at providing you with a competent product that YOU sale, you dont say well you need to go talk to the guy I buy my tomatoes from or the vender that supplies my store with faulty equipment....the customer is buying from YOU, which means YOU! Not Discovery Plus need to take charge in fixing the issue.
If you go to Walmart and buy a Roku and the Roku doesnt work right, do you demand Walmart fix the Roku?
At best, you return it for a refund, assuming its within the return window. Otherwise, no, of course Walmart doesnt fix or even attempt to fix the Roku that you bought through them.
(in fact, if they get enough returns/return percentage for a given product, they may give feedback/demand product changes to the manufacturer, or otherwise stop carrying the product)
The same is true for subscribing to Discovery+ through Roku Pay - Roku can cancel/etc your Discovery+ subscription that you obtained through Roku Pay, but it cant do anything about fixing the app, or addressing content issues within the app, etc - that's the entire purview of the app/service developer/provider: Discovery+.