Errors Using Bootable El Capitan Installer

hi! looking for some help resurrecting an old macbook, and have been running in circles:

macbook pro 17", around 2009/2010, was running El Capitan. hard drive is erased, no backup, no time machine.

on another machine (also el cap) i download OSX 10.11 installer dmg (from: How to get old versions of macOS – Apple Support (UK)), unpack, and use createinstallmedia on a USB drive as directed (here: How to create a bootable installer for macOS - Apple Support).

restarting the target machine with option key i can choose the installer disk to boot, get to disk utilities, but the install OS X fails with 'download cannot be verified/validated'.

i see some conflicting information here about the date on the certificates from reading forum posts, so try to repeat and set date in terminal to 2015, 2018 (different posts suggest either). same error. have tried with wifi off (but not simultaneously cable-connected to router).

booting in cmd+R on the native machine and selecting reinstall OS X i get error that some required download is missing.

i have tried making bootable high sierra but have a suspicion that the machine doesn't support it.

spoke to apple support, who ran me through some of the sme things as well as doing first aid on the HD (no problems, but no progress after).

would highly appreciate any tips or suggestions! i see some similar questions here that were resolved by what i think i have tried, but i'm not having the same result. thank you for any help!

np-junction :)