Thanks in advance !
To transfer assets from one network to another you are to use bridges. Personally I transfer from ERC20 to BEP20 and vice versa on https://bridge.evodefi.com/?token=USDC. The minimum here is only 50$, so I hope it will be suitable for you. Be very careful when it comes to different blockchains as if you send tokens that are not support by this chain, you will lose them.
The OP asked about swapping USDT (Tether) and not
https://bridge.evodefi.com/?token=USDT
As far as I know, this is the most popular bridge between Ethereum and Binance Smart Chain:
https://anyswap.exchange/bridge#/router
The minimum swap amount is 12 USDT. You need to connect to the website via Metamask.
I also know the following cross-chain bridge that can be used to convert ERC20 tokens to BEP20 tokens:
https://bridge.orbitchain.io/
The swap fee is 0.1%.
I found yet another multi-chain bridge:
https://o3swap.com/swap
Its developers warn about the beta version, so swap tokens at your own risk.