Your paragraph is hard to read without punctuations. Let me fix that:
Let me explain again: I have an isp provided router from where I get my internet now. The router which they provided is basic, so I have connected it to my openwrt box to get more advanced feature. But the problem is that when I am connected to the isp router, the ipv6 works perfectly fine, whereas when I connect to the openwrt box, the ipv6 doesn't work on the lan side of thing. The openwrt box is able to ping ipv6, but when I am on its lan ipv6 doesn't work though I get a ipv6 address from the openwrt box on my lan devices.
The root cause is your ISP router requires a prefix via DHCPv6-PD from your ISP, but does not allow LAN clients to require sub-prefixes from itself. In order to make OpenWrt's IPv6 routing work natively, OpenWrt needs at least a /64 prefix.
If nothing else is connected to your ISP router, consider setting it to bridge mode, and let OpenWrt handle the address assignment.