Can I use the 24h TWOV policy

Veronika, you can use 24 hour TWOV, but I will just repeat how you should calculate your entire transit time in China. So, your transit time begins at the moment of your first scheduled landing in China, in your case in Changsha, until your final scheduled departure, in your case from Guangzhou. So, if your entire transit time that consist of two layovers (Changsha and Guangzhou) and of a flight between Changsha and Guangzhou is less than 24 hours, you will be definitely eligible for 24 hour TWOV and you don't need any kind of visa.

You will print out your flight tickets and bring those printouts with you. Also, you will declare at check-in in Frankfurt that you will travel to Manila via China using 24 hour visa free transit. After a short check of your tickets, you will be allowed to board the plane. You will also inform yourself about the luggage policy, although if you are traveling on a single ticket, your luggage will be probably tagged all the way to Manila. Anyway, you should ask for this detail, just for a case.

Upon your arrival in Changsha, you will approach 24 hour visa free transit counter where you will apply for a temporary entry permit. This permit will be granted to you because your next flight segment is considered to be a domestic one. You will also clear immigration in Changsha.

After the formalities in Changsha are done, you will normally proceed with your flight to Guangzhou and later to Manila. Definitely visa free.