Toward the end of @carverofchoice’s post here there is a section you can expand to get the old unsupported RTSP firmware. If there is a version between that and the current one you want to get back, you can probably find that on something like waybackmachine or internet archive etc too
In the app under cam settings then firmware you should be able to uncheck auto update (not sure if the app even communicates with the RTSP firmware anymore though, but I’m assuming it does).