As you may already know, Wunderlist will be discontinued on the 6th of may 2020 and they are suggesting their current users to export their todos to Microsoft To Do (as explained here). It's not pure coincidence since Wunderlist was bought by Microsoft on June 1, 2015.
If Microsoft To Do doesn't suit your needs, here is an alternative : Todoist !
They also have a way to import your current Wunderlist todos into their app ! I'll let their website convince you completely, but let me tell you that their app achieved almost a parity of all features that made Wunderlist great (like : shared lists, sub-todos, reminders, phone + desktop app synchronisation, etc).
Some things are a bit different, but sometimes in a good way !
I would suggest you try it, and let me quote you that from their website :
In the 13 years and 71 days that we’ve been building Todoist, we’ve never considered selling out or becoming acquired.
Our team is committed to staying independent and earning your trust for as long as you need our apps.
Well, that's good news :)