pi zero 2w: ups hat no power with battery - battery issue?

hi - first post

i have this product: UPS Hat DFR0528

upon connecting to pi zero with a 2000mAh battery at 3.7v i'm unable to get to power the pi zero 2w. it lights up with 5v wall power and has 4 solid leds, and a blinking 5th. the signal light is solid both with and without the battery connected.

when i remove wall power and leave battery connected the board has no lights.

i've also pressed and held the function button for 2s and it changes to 6hz blinking on function led and then 2 leds on the battery status. again only with wall power since when battery is connected and wall power removed no leds are lit.

it's a version 1.0 board.

any ideas what to do here, could it be my 3.7v battery and its control circuit? this is the battery i have https://www.amazon.co.uk/dp/B08214DJLJ

also i have left wallpower and battery connected for some time to charge if needed but even after an hour it returns to nothing when wall power removed.

finally i've cycled the function and going to leave it to charge for a few more hours (i might be seeing some indication the battery is in a charging state) (will post findings in a few hours from now).

update: so the leds are gradually lighting up in the cycled mode, it was 2 leds and now 3…. however the function led is flashing at 6hz, which according to the documentation means no battery detected.

somewhat confused.

advice appreciated.

thanks.