How to derive public child keys from public parent keys with bip39-libs.js?
bip39-lib.js works along with bip32-lib.js AND bitcoinjs-lib.
BIP39 is strictly regarding the mnemonic phrase and HD seed.
In order to derive child keys from an HD seed you'll also need to use BIP32.
BUT... BIP32 has no concept of addresses either so ultimately you'll also need bitcoinjs-lib.
In other words you need to use all three libraries to go from mnemonic to a usable address.