First of all, I suggest to check if you plugged your Ethernet cable to correct router's port. It should always be WAN port to obtain internet access via wired connection.
Secondly, there could be an issue with DHCP server and you should try static configuration. To do that, go to Network > WAN and press Edit wired interface configuration:
Under General Setup tab select Static protocol, write down IPv4 address (your router's address on the WAN network), select netmask, write down IPv4 gateway (the address where the router will send all the outgoing traffic) and DNS address. Press Save. For reference you can use example below:
You can read more about static configuration: https://wiki.teltonika-networks.com/view/WAN