I suppose you have two different ISPs and are not willing or in the position to use BGP...
You can do load balancing by entering two (or more) gateways in the routing table/default route. You can do "weighted" load-balancing by entering a gateway address several times, so a default gateway of "188.8.131.52,184.108.40.206,220.127.116.11" should send out two third of your traffic to 18.104.22.168, and one third to 22.214.171.124.
This setup doesn't take into account that one uplink could be unavailable. In this case you would create big packet loss, as the router still would try to send out packets over a "dead" uplink.
To get around this you could use the "/tool netwatch" utility to monitor the two uplinks and use scripting to rewrite your default route accordingly.