was struggling with the same problem but I eventually figured it out. There is another requirement for recruiting Saias.
I got the following information from: (https://fireemblemwiki.org/wiki/The_Fiend%27s_Lair)
"If Ced is recruited or any of the Civilians are killed or captured, Saias will leave at the end of the chapter."
If any of the civilians are being held by enemies as you end a chapter Saias will leave. I replayed the chapter while saving the civilians and was able to successfully recruit Saias.
ALSO Saias won't appear in your character deployments options for chapter 24 even if you success recruited him. He will be deployed automatically at the start of chapter 24.
I didn't find this information explained clearly anywhere so I made an account just to post it here for future players.
Rare
