This particular site has very difficult link conditions - no los - and more bandwidth is required. I decided to try bonding but I don't know which mode to choose for bandwidth aggregation between 192.168.1.1 and 192.168.1.2. The AP (WDS) has enough bandwidth for aggregation, but so far I managed to setup successful failover only. Which bonding mode allows bandwidth aggregation in such scenario?
Bonding Wifi signals is a shaky proposition to begin with; when you throw in a poor signal, it's just a bad idea. The reliability of a bonded connection is roughly the product of the reliability of the bonded links (e.g., 80% x 80% = 64%).
If you want more effective bandwidth, the first thing you need to do is figure out what it takes to establish one solid, reliable link. That may mean using 900MHz and high-gain antennas, or placing a relay station somewhere that has LOS to both endpoints. Once you have that, you can increase capacity by adding more links of the same kind and load-balancing between them. Each connection will be limited to the speed of one link, but you will be able to support more connections in parallel.