Unable to connect to some websites from legacy Kindle

I am having an extremely weird issue, regarding an old Kindle 5th Generation (non-touch).
For whatever reason, I am unable to access either DuckDuckGo (attempted to access DuckDuckGo Lite) nor CloudFlare from it’s browser, however I can access Google(?).

I can say that I have no such issues with any of the other devices in the house (Android devices, Windows devices, Linux devices), so I have no idea what is going on here.

And yes, it is on the latest version, 4.1.4, and is connected to the wireless network (probably via WPA, have not yet checked whether 4.1.4 added WPA2 support).
The routers are a mix of OSs (this is temporary):
Netgear DM200 (for connection to ISP): OpenWrt 19.07, with updated xDSL firmware to enable vectoring).
Xiaomi Mi3G V1 (Main, to be replaced with a LinkSys Wrt32X in the upcoming weeks): PandoraBox (I forget the version, a derivative of OpenWrt, to be replaced with OpenWrt 19.07)
Xiaomi Mi3 (serving as repeater, to be replaced by the Xiaomi Mi 3G): X-Wrt (again, I forget the model, however this is pure OpenWrt Dev with a preinstalled GUI).

The Kindle was connected directly to the router, and the router is configured to use (IPv6 is disabled since the network operater (the ones who supply the infrastructure to all of the ISPs here) thinks that it is unnecessary (idiotic, however currently I have nothing else to connect via IPv6).

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