Technically you can use your vertical digital planner in landscape mode, but I do not recommend it. If you rotate the planner the design will not adapt to fit the page. You will need to zoom-in/out to access all the sections, and it will be a bit annoying. Why not try the dedicated landscape version instead?
Keep in mind that:
a. the new iOS13 allows for Split View (see 2 documents simultaneously) – in this case the Vertical iPad view is better for you
b. if you are using your tablet horizontally, connected to an keyboard (similar to a laptop), then the landscape iPad view is better for you