President Trump cannot disband a federal grand jury. The grand jury is protected by a chief judge of the U.S. District Court for the District of Columbia who has the right to defend the integrity and independence of the U.S. District Court.
The U.S. District Court could appoint a new prosecutor if Robert Mueller is fired, then the Mueller grand jury will continue its plight of investigating the Russian interference in the 2016 election.
If push comes to shove, the U.S. District Court judge could proceed with all pending investigations, and then make all findings available to the public. A president cannot meddle with a grand jury, reason being: The grand jury occupies an independent niche. Love those niches!
Consequently the beat goes on and it’s still Mueller Time.