Electric shower issue

Hi all about 2 years ago I had a brand new electric shower fitted although I’m quite comfy with plumping just did not wanna mess around in the loft so we had a plumber fit it for us and he was newly qualified but once fitted we only got a dribble from it and he said it was because we had low pressure so it just sat that doing nothing but annoying me, so I finally purchased a shower booster pump as heard good things, went to install in and realised he did not plump it into the mains but a t off from the loft tank that feeds the upstairs taps gravity fed I believe it’s called? Well I fitted the pump to the pipe that feeds the shower and the pump is power by mains socket it lights up blue to say it’s on but nothing is happening and still a dribble? Am I missing something.

pump is this one https://showerpowerbooster.co.uk/gravity-fed-systems/

No idea what you have done without seeing your set up. However ,if you turn on the shower to start up ,does water come out at all ?
You would be better off connecting the shower to mains water supply ,it simply will not work on gravity set up .The pump will not start until water flows ,and you don't have the pressure .