Find My Phone shows "No Active Devices" I have a Moto G5, running Android 8.1.0 (the latest available version for this device). I previously used "Find my phone" and it worked. 

I had to factory reset my phone.

Recently I wanted to use Find my Phone again, but I noticed that when I went to google.com/android/find it shows "No Active Devices".

Luckily I found my phone, but I wanted to make sure it would be findable by find my phone so I walked through the steps at https://support.google.com/android/answer/3265955 

Confusingly, the device is turned on, it's signed into a Google account, it's connected to both mobile data and wifi, it's visible on Google Play, it has Location turned on, and it has "Find My Device" turned on.

If I go to (on the phone) Settings > Google > Device Phone Number it says "no number available", but the device correctly shows up on the webpages when accessed on pc. Also, if I go to Settings > System > About Phone > Status > Sim Status I expect to see my phone number listed, but it says "unknown" for My Phone Number. If I open Contacts it correctly lists my number, although I had to add this. If I go to https://myaccount.google.com/phone it correctly shows my number, and that it has been verified. I'm not offered an option to re-verify it.

I have downloaded the android app "Find my device" to my phone, and the app will show my phone as an active device and will ring it.

What do I need to do to make sure my phone is shown as active in the "Find my Phone" webpage?

(I don't know what appropriate tags would be).
I was able to install the app "Find My Device" to the phone. I then used the app to "secure my device". This caused my account to be signed out from the device. I had to sign back in. My device now appears in the Find My Device webpage.

This didn't fix any of the "phone number not available" settings, so I guess they're not relevant.