NOT_FOUND for send means: Reported Object was not registered to Server
So we should rather use BAD_REQUEST which means "Undetermined error occurred" See : https://www.openmobilealliance.org/release/LightweightM2M/V1_2_1-20221209-A/HTML-Version/OMA-TS-LightweightM2M_Transport-V1_2_1-20221209-A.html#Table-67-4-Response-Codes-Information-Reporting-Interface