Never mind my first answer. Let me explain this better

On trust wallet, you can use a particular chain to buy coin built on that chain, for example, you can buy TUSD which is built on ethereum blockchain with Ethereum.

So you can buy BEP2 tokens with BEP2 BNB

Which also means you can buy BSC tokens with BSC BNB.

You can convert BEP2 BNB to BSC BNB, but you can not convert BEP2 BNB or BSC BNB to ETH.

If you have BSC BNB, you will still not be able to convert it to ETH to be able to buy ERC20 token.

If you only want to trade $5.1, I will advice you not to trade it because it will required you to first go to am exchange that support conversion of BNB to ethereum like binance, then moving your ethereum to the wallet back, then swap it for the ethereum token. The transaction fee for ethereum on exchanges is huge, over $20 ethereum fee on binance.