I read that
HP Laptop>HDMI-HDMI>Sony TV OK
Dell Laptop>HDMI-HDMI>Sony TV FAIL
Dell Laptop>HDMI-HDMI>Panasonic TV FAIL
that using the same HDMI cable and the same way round?
Do you have another HDMI cable you can try?
Is there a HDMI switch or ext monitor keyboard switch on the Dell that is set=OFF.
See this: My second monitor is not getting detected - #12 by citfta
What model is the Dell Laptop? Dell Inspiron 3552 , HDMI version 1.4a