Assign LAN port as WAN on Technicolor TG582n router
Yes, it is possible. I'm not sure whether this can be accomplished with the stock firmware though, as I did it with LEDE/OpenWrt.
Basically, I created two virtual LANs (VLANs), one for port LAN 1, the other for ports LAN 2-4. Port LAN 1 in the first VLAN works as the WAN port, using the PPPoE protocol to communicate with my DSL modem. The other three ports in the second VLAN serve as standard LAN ports for my local network. The specific steps needed are detailed here: https://onetransistor.blogspot.it/2017/04/wan-port-openwrt-lede-vlan.html