After having massive connection issues for days (no connection at all, connection but no route to gw, flaky connection e.g. termination after a few minutes), I rolled back to FW 1.13.3 and got a stable connection on an RUT240 and RUT230 device, even with 4G (could only establish unreliable 3G before). Location of the devices and antennas was not changed in between.
(edit: inserted the missing 1 in the subject title, 1.4 -> 1.14)