DeNA have announced the details for the next Champion Stadium challenge which will be Kanto and it takes place from January 31st, 2021 at 10:00 p.m. until February 28th, 2021 at 9:59 p in Pokémon Masters EX.

The Next Region for the Champion Stadium

The details for the next Champion Stadium challenge are planned as follows.

Kanto Challenge

In the Kanto Challenge, you can go up against the Elite Four and Champion of Kanto during the time period listed below.
Note: For the duration of the Kanto Challenge detailed below, you will not be able to take on Champion Stadium battles for other regions.
Note: Medals for regions that are not currently active will be available during the next challenge that features those regions.

Kanto Challenge Date/Time

January 31, 2021 at 10:00 p.m. - February 28, 2021 at 9:59 p.m.

Important

• You can participate in the Champion Stadium after you complete Main Story Chapter 10: A Reason to Battle (Normal) in single-player and collect all PML Badges.
• Medal acquisition and Hall of Fame information will be saved for each region regardless of event date/time.
• Details about the time period and featured region will be included in notifications going forward.
• The time period and featured content are subject to change without notice.

Source: https://pokemonmasters-game.com/en-US/announcements/Update_2120_4W_4