maybe someone can assist better, if you post the OpenWRT network config file formatted according to How to format logs, scripts, configs and general console output
The part where I can help here a little: do not assume, that prefix delegation through OpenWRT is autocomfig-based or that it selects a working subnet automatically or that it is able to pick the optimum on itself. I once tried PD and got the impression that I have to manually try different prefixes and it can end lots of manual trying different prefix lengths on WAN and LAN side. And any prefix setting „AUTO“ value more like means „default“ (I cant remember, what default value that would be)