A lot of small routers support Openwrt but there might be some quirks with some models out there. In that case, you might like to revert back to original/stock firmware. This blog post talks about steps needed for that.
Access Point setup is needed to cover a space that can not be entirely covered by one WiFi device alone. This blog talks about setting up Access Points with a Griggi router but steps are generic enough & will work with any router.
Some of the advance features that Griggi router support need manual configuration. This post talks about those steps.
Griggi can be installed on any Openwrt router. Follow the steps on this article to do so.
Steps to setup IoT devices with Griggi as Griggi by default blocks internet for any device that can not do manual authentication