Your hardware might not allow you to upgrade to Mojave, but I suspect you can still upgrade from El Capitan to at least Sierra (10.12) if not High Sierra (10.13). I suggest you upgrade OSX and see if that solves your problems. After all, El Capitan (10.11) came out in 2015 (?) and Apple stopped supporting it last year (2018).
@Paul_Allen This thread should probably be merged w/the “Blank Screen” one.