Replace custom ROM system app with stock variant

I am running Resurrection Remix Oreo (8.1) on my Samsung Galaxy S7 (herolte). This has been my preferred custom ROM for years now. However, I have an annoying problem: the SIM toolkit[*] application does not work properly. It used to work on the phone I had before (Galaxy S4 with RR 8.0). For testing purposes, I switched back to stock firmware and gave it a try. Everything worked fine. Thus, I would like to copy the original STK app to my custom ROM. I also tried RR Pie (9.0) and original LineageOS 16, without success. However, I cannot just copy it over. It somehow must be signed correctly. This is my first problem. Can I sign it with the public android platform key? Or would it have to be signed with the RR developers' private key? There is another problem: The app is made for Android 8.0 (API 26) whereas I am using Android 8.1 (API 27). I might be able to change the manifest using apktool or something similar. I would appreciate any advice. [*] I use this as an authentication method for my online banking platform. The bank sends a message to my phone telling the SIM card to pop up a dialog where I have to enter a special PIN (not the normal code to unlock the SIM card). The phone then sends back a message to my bank telling them to log me in. What currently fails is the "OK" button. Pressing it gives absolutely no reaction. In more recent versions of LineageOS, there is also a "Cancel" button, but it does not work either.