What happens if we don't elect a House Speaker by January 6th?

[Polybius]

The initial vote of the newly convened House, scheduled for January 3, will focus on the election of a Speaker. Subsequently, on January 6, the House is set to convene to certify the outcome of the 2024 election.

It is important to note that no legislative activities can take place in the House until a Speaker is appointed, and the certification of the election must occur on January 6. This raises the question: what are the implications if a Speaker has not been elected by that date? Could this potentially postpone the certification of Trump's election victory until after the 6th, but before the 20th? Furthermore, what would be the consequences if a Speaker remains unselected by January 20?

That seems to indicate that the President Pro Tempore would become Acting President, since that would be the next position in line. Biden and Harris will be out no matter what on noon of the 20th.

No President, no VP, no Speaker. That means (wait for it) 91 year old Chuck Grassley would become Acting President. And I thought Trump was too old...