Looks like the 86" big screen LG TV downstairs support subtitle via Windows Media Stream. But these subtitle need to be "embedded" in the video and in this case, MP4 format. I am not sure about AVI. Media Stream subtitle doesn't seem to work well with subtitle files separately.
To do this, thanks to this site:
ffmpeg -i infile.mp4 -i infile.srt -c copy -c:s mov_text outfile.mp4
Update 1/11/2024: For hardcoding subtitle to mp4, this online free tool seems to work well, although it turned my 2GB mp4 into 3GB mp4.