Please test with your box and TV remote while connected to HDMI, and provide TV make and model build date and S912 box make and model with firmware make.
Here is my fully functional IR TV remote device info.
Samsung 2008 DLP RPTV Model HL61A750A1 with Alphawise H92 Pro Plus LPDDR3 3/32 with latest stock build (aml_7.1_s912_q201_q9377_H96PROPLUS_LP-mac-20180706) lightly customised by me.