Do this in turn: after inserting the page number into the whole document, put the cursor on the next page under the page layout-separator drop-down menu on the second page (as shown in figure 1), put the cursor on the footer of the third page, and uncheck "Link to previous page header" under the Design menu (as shown in figure 2), which is divided into two parts. Then put the cursor on the next page under the page layout-separator drop-down menu on page 4 (as shown in figure 1), put the cursor on the footer of page 5-uncheck "Link to previous page header" under the Design menu (as shown in figure 2), and then move the cursor to the page number of page 3. Select a page number with the mouse, click Insert Menu-Page Number-Set Page Number Format, select "Start Page Number" in the pop-up "Set Page Number Format" and fill in "1" manually, and select the I and II formats you want from the numbering format options. Similarly, move the cursor to the page number on page 5, select the page number with the mouse, click Insert Menu-Page Number-Set Page Number Format, select "Start Page Number" in the pop-up "Set Page Number Format", manually fill in "1", and select the desired format from the number format options.
Of course, according to your needs, the page number of the first page 1-2 is unnecessary. You can select any page number of 1-2 and delete it. Don't worry about being affected in the future. Because they have been separated by section break.
It's actually quite simple. I wrote so much to make you understand. In fact, the key is to master the usage of "section break" with a few mouse clicks.