If someone promote "no fees" for an ERC20 swap on a DEX, they are lying. It is not possible as it is running on the ETH network, which uses fees to run...
Using a DEX for a swap is interesting when it has enough liquidity to avoid big slippage. To attract this liquidity, DEX put some additional fees on swaps, fees they redistribute to the liquidity providers!
So you have ETH fees for running the swap smartcontract, + DEX fees.
Here is a little comparison of additional fees between the different DEX
You can see uniswap has 0.3% additional fees, and defiplaza.net only 0.1%
I like this defiplaza DEX as even its smartcontract is optimized for low fees swap.