Hoping someone who's more familiar with HSA rules can advise me on this time-sensitive question:

End of this month, I'm switching from a HDHP to a regular plan, which will make me ineligible to continue making HSA contributions going forward. I've been making bi-weekly contributions to my HSA from my paycheck/severance, but I only reached ~50% the yearly limit. Am I able to make a cash contribution before the end of this month to "close the gap" or can these contributions only come from the paystub directly?

Not sure if it matters, but perhaps it does since Fidelity seems to allow for cash contributions - my HSA is right now within my employer's benefits account but I'm in process of rolling it over to Fidelity for more control & wider investment options.

TIA