I never heard about Tatum wallet but from what I see this is not free service or it can be free but with some limitations, and one of them is that you can't adjust fees.
You would need to purchase Enterprise package to get this option enabled (that is more than 49$ per month you need to pay for Basic package).
You can use Electrum wallet with testnet with manual fees, other wallets with this option are Coinomi and Trezor hardware wallet.
If you want to check fees for Bitcoin testnet before sending transaction check out mempool.space website:
https://mempool.space/testnet