Your imported address(es) are not backed up by your seed phrase. Your seed phrase can only generate addresses belonging to your HD wallet.
If you have the private key, as mentioned by LoyceV, you should create a new wallet in electrum and select "Import Bitcoin addresses or private keys".
Note that you need to add a prefix to specify the address type when importing the private key into electrum.
If your address starts with 1, use the prefix "p2pkh:".
If your address starts with 1, you can also enter your private key without any prefix.
If your address starts with 3, use the prefix "p2wpkh-p2sh:".
p2wpkh-p2sh:YourPrivateKey
If your address starts with bc1, use the prefix "p2wpkh:".