Cellular links will have variable latency, you can mitigate that using a latency cut off setting in the profile so that when a link latency ramps up it is temporarily removed from the bonded tunnel until it recovers.
Take a look at his post from when that feature was introduced for more info:
Are you using fixed Public IP SIMs in the HD4? If not its likely that carrier grade NAT is getting in the way of inbound traffic to the cellular connections so even though the link is online with working internet access no traffic is allowed inbound so the individual tunnels can’t be built.