IPFire supports various connection types to connect to the Internet. They all differ in the way of being set up.
This is the most simple mode, where IPFire is connected to an Ethernet network and is assigned a static IP address in the same network. Most commonly this is being used in data centers.
In this mode, IPFire is connected via Ethernet to a network and is leasing an IP address over the DHCP protocol. This is most commonly used for Fibre and Cable connections or if IPFire is running behind another router.
Note that some ISPs will call this "IPoE".
In order to connect to a DSL provider, IPFire needs credentials to connect.
With an appropriate wireless modem, IPFire is able to connect to a mobile/cellular network.
No, this is best done on layer 2 of the network and in conjunction with the ISP. Using multiple Internet connections at the same time is theoretically possible, but in practice creates a large number of new problems.
IPFire can however fall back to a backup connection in case the primary connection fails.
Some providers will need some extra settings or tweaks to function properly with IPFire. Here is a collection of those: