Hi, Please try to update the BIOS from this link. Upon reviewing the case, I did notice you had mentioned when you uninstalled SupportAssist, it re-installed on it's own. We are not aware of a self-install unless you have one SupportAssist installed from our Support website and the other installed from Microsoft Store? I've seen an instance where there are issues if multiple applications for SupportAssist are installed. It is always advisable to have only one installed on the system. 
Dell Support assist says this is an urgent update but won't update on my Latitude 5290. Now what? This has been going on for a month.
Dell Latitude 5290, 5490, and 5590 System BIOS