iCloud backups of iPhones can be done without a paid iCloud+ subscription, provided there is space available on the iCloud account. It is advisable to have much more space than you need currently and an iCloud+ plan is a wise investment. However, if you don't have your iPhone you can't back it up.
Backups require that you plug it into a power source and have Internet access - and have the iPhone set for iCloud backups. Unless you have left your missing iPhone in this state it won't be backing up.
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