I dont think that is going to be the problem because I can establish a link with the new 126.96.36.199 and another 188.8.131.52 that I already had. but I can not establish a link between the 184.108.40.206 and the 15.0.3 unit. Both new modules and both came in on the same order.
I have confirmed that this combination should not work--13.3 has a change to get more throughput that produces an incompatibility.
We need to update the documentation to show this upgrade path.
In the mean time, if the devices are already installed, and you have 13.foo.bar at the far end and 15.splee.spline at the near end, you'll have to downgrade the near end to 13.foo.bar, upgrade the far end to 15.splee.spline, then upgrade the near end to 15.splee.spline, and they should talk. (Otherwise, if the far end is new (15.splee.spline), just upgrade the near end to new, and you're good to go.)
I apolgize for all of the problems you've been having.
You can use CNUT 4.10.5. You can donwload it from the link below. Procedure to downgrade is same as upgrade. You just need to select the correct package on CNUT to which you are downgrading or upgrading.