Coinomi is not using Algorand 25-word mnemonics.
It is instead using their own version of BIP39.
They provide a tool to recover the Algorand secret key from their 24-word passphrase:
https://github.com/Coinomi/bip39-coinomi (you need to download the zip file from the release page and open the file bip39-standalone.html from the zip file.)
Important note: I did not review the security of the tool above nor of the process below. You are fully responsible to review that both satisfy the security guarantees you need.
After entering your passphrase, you will get a list of Algorand addresses and secret keys (like 0d9cbee592b839f58814b84a77122d3d5939e745c328de99851a617b82e02598)
To recover the normal 25-word mnemonic from the hexadecimal secret key, you may run in the terminal:
$ echo -n 0d9cbee592b839f58814b84a77122d3d5939e745c328de99851a617b82e02598 | xxd -r -p > key
$ algokey export -f key
Private key mnemonic: like whip index matrix inflict diary picture retreat true endorse notable need increase keep boss churn design biology minute render agent join scorpion above roast
Public key: ZDGCBU2U2IEQWLXKP7G77XZWOXNYON4DQXZKEDYLUBJ4XR2UYNMF74TIP4
Disclaimer: Typing passphrases and private keys on a computer may open you to attacks that are not possible on the Coinomi hardware. Algorand makes no representations regarding the functioning or security of third party wallets, exchanges, or tools mentioned in the answer above, and disclaims any liability therefor. You are fully responsible for the security and the availability of your keys.