3WDS with distance of 30Km: Point to multipoints work bad

I implemented a lan like point to multipoint, where there is an AP-Brige and 3 remote users far aboute 20 km.. The 3 different users are connected with 3 different WDS with tables of routing without forwading.
I have big problems:
-lose of pings;
-low bandwidth about 4Mbps in test tcp-both direction, (I have all antenna with 24dbi and all router with 30dBm power);
-sometimes the link goes off: not stable link.

Someone know how can I configure the this LAN?
Please help me



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