How do I reinforce the seats on a wicker chair?

Lorianne some people use a vinegar/water combo on sagging wicker chairs. After applying place outside in the sun until they are good and dry. As for the hole you might have to do some cane weaving. Here is a site that explains that process. https://www.wickerwoman.com/articles/chair-caning-instructions

Maybe you can try to use some twine under it ...just wrap the twine around the base and go back and forth under the chair where the small wear it at ....I don't think the Twine would be very visable ...it might help ...its just a suggestion you might want to give it a try ....

You can go here for help https://www.wickerliving.com/blog/10-steps-to-repair-missing-or-damaged-wicker-furniture-reed/

I redid mine with some rope that looked like rifia that I got at Lowes. I used a " mattress" needle and wove in between the wicker and pulled it tight and tacked it in back. It turned out great. Then I painted the chair white.

I would try to reinforce it from underneath with a piece of wood.

They make plastic wicker that is of course water proof and easier to keep clean.