Hi Igor,
We are also having this problem where the RUT240 is not returning to the Main WAN after it is available, it will remain on the 4G Failover.
When we disconnect the 4G Antennas, this fails the 4G connection and the RUT240 will use the Main WAN.
However it will not switch from the 4G to the Main WAN if the 4G connection is working well.
This is a large problem as 4G data is quite expensive and we want to use it only as a failover for customers not for extended usage, we have done a test and for over 30 minutes there was no switch back to the Main WAN.