Hi:
Below is the link to the service manual for your notebook.
HP EliteBook 828 G3 Notebook PCHP EliteBook 820 G3 Notebook PC Maintenance and Service Guide
You can only install one of the two WWAN cards listed in chapter 3 page 19 of the service manual.
(9) WWAN module
HP lt4120 LTE/EVDO/HSPA+ SnapdragonT X5 LTE Mobile Broadband Module 800870-001
HP hs31110 HSPA + Intel Mobile Broadband Module 822828-001
You must buy one that has that HP part number on it.
Search by the HP part number, not the model of the wifi card.
I would say the best place to see if you can find one would be on eBay or Ali Express.