I have a Kobo Nia and live in Shanghai. I sideload my books so don't use the Kobo store but I can say the OS update server is blocked so I suspect the store is too. The solution is a VPN / Proxy that is on a device your Kobo connects to. At least in China lots of very cheap and cool little WiFi routers like the
Cudy AX3000 will provide VPN to a device like a Kobo.