I haven’t been able to find a way insert a page break with a keyboard shortcut. Is the only way by pull down menu Edit > Insert > Page Break ?
you can set your own shortcuts. Find details here: https://forum.literatureandlatte.com/t/change-assignment-of-keyboard-shortcuts/39753/1
There is also a knowledge base article on the matter, and Appendix A.1 in the user manual goes over the technique, if you prefer offline instructions.