I bought some USDC and sent them to my Coinomi wallet, but they have never appear on the wallet. I have no way to use them, despite I can still see them on the Etherscan.
Are you using the mobile wallet or desktop wallet?
Is your wallet fully sncyed?
From the information I can see, the support USDC. If the problem persists, try contacting their support -
https://coinomi.freshdesk.com/support/home