Can you please turn off your Turris MOX and plug the SFP module only when Turris MOX is powered off and then turn it on? There is a possibility that hotplug for SFP is not supported.
Turris OS version 4.0 is no longer supported and you should be using the latest major version, which is Turris OS 5.0, which is based on OpenWrt 19.07 version.