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Failover feature allows you to have backup WAN connection, in case if primary WAN connection goes down. As mentioned above, router could not provide internet access when there are too many users connected at a time. But primary WAN, which in you case is mobile WAN is still running. Thus, its an issue of bandwidth, not of connection going down.
In order to resolve this issue, you can use load balancing feature of the router. This feature will split the traffic of the users on the different WAN interfaces of the router.
Please note that you can either use failover or balancing one at a time. Both features cannot be used at the same time. Please follow the above link to learn how to configure load balancing on your router.
Apart from that, if you want to change your primary WAN connection. Enable the advanced mode on WebUI of RUt240 and go to Network > Interfaces, there you will need to drag & drop each interface to change its priority. In your case, you will have to drag the interface wired WAN interface at the top. Please see the screenshot below for better understanding:
Here, you can see that WAN (Wired ethernet) has been set on the top of the list, which represents the top priority of the interface. Following WAN, mobile interface and LAN come after the Wired WAN interface. Wired WAN will be the primary WAN connection in this case.
For more information about Rut240 Interfaces, please follow the link below: