Maybe the wallet doesn't support private key imports.
I don't know much about Monero, but maybe this can help > https://docs.featherwallet.org/guides/create-view-only-wallet.
It talks about creating watch-only wallets. That's not what you want, but it's not why I am mentioning it. At the bottom, it describes how to import key images. It tells you to go to
Maybe there is an option there for private keys instead of key images.

It's not that big of a deal, I think. We all know how difficult it is to find good multi-coin wallets. The Unstoppable Wallet that keeps getting mentioned probably doesn't even support 1/5 of the coins and tokens that Trust Wallet, Atomic, Exodus, and similar closed-source alternatives do. Then, what else is there than to suggest, mention, list, or recommend the worse options?

Unstoppable wallet takes too long to sync, and downloads hundreds of MBs (maybe up to 1-2 GBs) of data during that process for each coin. For many people with limited phone storage, that might not be the best choice.