A common issue with these older systems is the PRAM battery needs to be replaced [product|IF186-020] and here’s the guide to get to it [guide|297]
What happens is the primary battery was disconnected so all of the PRAM data was lost as the PRAM battery is not working. You could try doing a SMC and PRAM reset with the primary battery installed and plugged in (make sure the battery is charged). Do the SMC reset first !!
[https://www.macworld.com/article/2881177/how-to-reset-a-macs-nvram-pram-and-smc.html|How to reset a Mac’s NVRAM, PRAM, and SMC]