Hello, some more options to handle mobile connection is available under
Network > Mobile Tab.
Like Force 4g or Re register every n seconds, are you try to set this options?
Will be a configuration mistake, you can try to set differents combinations but the first thing when request advices, is posting your configuration tests and your logfiles from
System > Administrator Troubleshoot tab.
Here you can select verbosity of logfiles (set INFO will be enough for starting debug) and include (checkmark) all gsm/pppd/modem performing actions into log (System and Kernel) files.
After adjust and SAVE troubleshoot tab, you are ready to dump "errors", wait a while and get a look on your logs, post here as well.
This information will help much more to diagnostic issue, than a human description meow
Also you can grab acutal configuration easy, by downloading support files (Troubleshoot files button)
Hope this will be helpful! A side note, my rut240 after firewall fine tuning works smooth and faster like a rocket