First you need to plug into the fiber box directly without the sitecom. You need to be sure the ISP has everything right. Even a crappy router should get 250m on wired. I know nothing about the sitecom box but it likely uses the same chips as many other routers so it should perform ok. There are only a couple of chip manufactures that all router makers use.
You should have no trouble with any of the major brands. Asus,tplink,netgear etc. Stay away from "value" brands like belkin,trendnet,buffalo....mostly because their support is not as good if you have problems.