I downloaded the mirror app for Roku but it will not connect completely using iPhone or iPad. There is no clear picture or sound when connected.
Its not clear what "mirror app for Roku" you downloaded from the Apple store (there are plenty).
You'll need to be more specific.
Also be aware that newer 4k/HDR devices now have AirPlay2 support, and considering that the 3500 is nearly 7 years old and less capable, you might consider getting a newer model (for $30-50) just for the AirPlay2 support:
https://support.roku.com/article/360057488733