Ive been out for awhile, I have 320 mithril coins and the fee to unlock is 15 gold and 525 silver. It says "locked until restoration fee is paid" but the mithril coin button just says "Get more coins" as opposed too "Mithril deposit upkeep"

I guess what I am asking is why cant I pay my restoration fee with mithril coins? Is there a way to convert mithril coins to gold?