Dounan bus from Chitose to Noboribetsu

Hello. I am going to Hokkaido this April, and my flight to Sapporo arrives on April 26th at around 10.30 in the morning. My first stop is Noboribetsu, and I found out that the best way to get there is by bus. I notice that reservation is a must to board the bus. How many days in advance could we book the bus? And if the reservation can't be made online for Foreigners, can we still book it at the airport by the time we arrive? I'm afraid that if we don't make an advanced reservation, we'll miss the bus.

Oh, we are staying at a Ryokan in Noboribetsu, and I've searched online from Japan Navitime that the best travel option is by Dounan bus at it stops at a certain hotel that is walkable from our Ryokan.
Thank you for your help.

How many days in advance could we book the bus?

I recommend to make the reservation after arrival in New Chitose, especially in case you arrive with an international flight. This is because with flights, luggage pick-up and possibly immigration it is never too clear when exactly you actually arrive at the airport and be able to ride the bus.

In the worst case (bus is booked out) you could take trains instead or a later bus (New Chitose is a pleasant airport).

Or you could make a reservation for a safe departure and plan from the beginning to possibly spend some time at the airport (or try to change the reservation to an earlier departure after arrival in Hokkaido).

How to travel between Chitose and Noboribetsu:
https://www.japan-guide.com/e/e6753.html?aFROM=6750_chitose

Thanks a lot Uji. You've been a great help not only to me but for others who are going to Japan. :)