Hi, there are some info about PPPoA at openwrt Network configuration [Old OpenWrt Wiki]
First you need to install ppp-mod-pppoa, then you can go to advanced settings (luci) to set things up.
For your 2nd question. If you click New connection, you will be able to set up one of the four methods: cable, repeater, modem and tethering. 4 methods can exist together. For cable, there are DHCP, PPPoE and Static. You only can select one method for cable connection.