Your backup phrase is Hierarchical Deterministic, meaning it is a seed key for many hierarchical and sequential bitcoin addresses. And each bitcoin address has their private keys. Your problem maybe not knowing the correct derivation path for Omni Bitcoin addresses. I remember it's BIP44 or m/44'/0'/0'/0
Check it. Do the below again.
Yes. That's all you need.
1. Go to
https://iancoleman.io/bip39/ (or download its source code and run locally for better security);
2. Write your recovery seed in the "BIP39 Mnemonic" field;
3. Scroll down a little bit and find your address in the list (the one you sent the USDT);
4. When you find it, copy its Private Key;
5. Now go to
https://www.omniwallet.org/ and create an account;
6. Log in, go to My Wallet -> My Addresses;
7. Click on the "Add Address" button -> "Import Address with Private Key";
8. Paste your private key from step 4 -> "Add Address".
9. You should see your USDT balance and be able to send them somewhere else.