Same happened to me. I wrote to Specialized to ask about it and they said the 520 plus is not in the list. Like you, I thought that as the 520 was listed the plus would also work. Luckily I managed to return the 520 plus and buy the more expensive 820, so it’s cost a lot of money to be able to see what mode I’m in and battery left. However, now that I have mastered the Garmin, I’m finding the information useful and more reliable than Mission Control. The mapping is also good. The good news is that the 520 plus is listed as being compatible with the ebike app which, according to this forum, is better and more reliable than the Specialized Levo app.
Download Jan's eBike Field app... test it out on a couple of runs then donate to help with its development. Great app that's way better than the Specialized one.