This article gives you a clue, how LAG / Link Aggregation on a NAS is working and which Port Trunking settings you need to setup.
Link aggregation combines LAN1 and LAN2 into a single logical link with greater bandwidth. Both Interfaces use the same IP address and MAC address. It also includes fault tolerance and load balancing. Connect LAN1 and LAN2 to the same Ethernet switch. Please make sure here that the switch also has the correct Link aggregation settings.
The option to set up the load balancing can be found in Control Panel > Network > TCP/IP > Network Interface > Create
If you want the NAS to always get the same IP address via DHCP, this, unfortunately, does not work as it is a limitation within LACP. It is recommended to set up the IP to static in this case.