I’m using the Slate AXT1800 on V4.2.0.
It’s possible to install the NextDNS CLI on this version, however it’s a bit dated. Currently on your repo the version is 1.37.11, and the latest is 1.39.4.
Could you update the plugin to the latest version?
When I tried doing it manually by uploading the IPK in LuCi, it showed me that the gl.inet repo told LuCi the app was already running the latest version (which it was not).
It looks like you’ll need to upgrade libstdcpp at the same time. Unfortunately, openwrt doesn’t come with a corresponding installation package, so you’ll have to wait for the openwrt version to be fully upgraded before you can use the latest nextdns.