Hi @VincentTanKS ,
I’m late to the party but here are the steps I followed:
1- in the Nextcloud Tasks, look in the left side bar. Create a tasks list if you don’t have any, and click the 3-dots button on any task list. In the options, there is an entry “copy private link”, click it: the link is now in your clipboard, keep it somewhere because you will need it on your phone.
2- In the tasks app, click the options menu (3 bars in the lower left corner), and select “parameters”. Click “add an account” near the top. In the options proposed, select CalDAV (I think you can also use DAVx5 but they say some features are not supported). Then, you have to fill in your nextcloud credentials: username, password, and the URL that you got in step 1. In “server type”, select “Nextcloud”. Save the settings (with the “save” icon on the top-left corner).
Hope this helps !