Bonding WAN connections is one setting allowed by ZeroTier’s “multipath” support. They have a “balance-aware” setting that may be the default – but we don’t know & without controls for multi path, you’re can’t optimize known link inbalances. You need the access to ZeroTier’s configuration file.
@anav, you asked a related question, “EMULATING peplink BONDING with RoS”, which also highlights another long stand grip:
See http://forum.mikrotik.com/t/emulating-peplink-bonding-with-ros/179605/1
So this one is a big +1 from me. We have any entirely duplicate WinBox , with lot of attention to detail – but somehow no-one at Mikrotik can find the time to wrap the ZT config as commands? So peplink sometime gets my [customers] money, which doesn’t have ZeroTier but has controls for asymmetric WAN bonding at 3x the price.