closing costs on paid off/refinanced loan

I refinanced a rental property in 2016 and have been amortizing the points properly on the refi loan since then, no issue with that.

However, I just realized that I didn't claim the remaining amortizable costs on the *previous* loan that was paid off by the 2016 refi - I just stopped amortizing the prior loan's closing costs that year, and there's a significant balance left that was never amortized/deducted :( 

Of course it's too late to claim a refund for this 2016 activity at this point.  Can I add the remaining (never-claimed) closing cost balance to the basis when I sell the property, or is there now nothing I can do with the closing costs that were paid for the previous loan but never amortized/deducted after the loan was paid off?

Thanks for any advise/suggestions! 🙂