Nighthawk R7100LG

Hi

I am a happy user of the Nighthawk. I wanted that my cabled net connection would be the main stream of Internet and in case of failur or weak signal the LTE card will be invloved. I installed it and it works perfectly with the net from my existing modem. On the website it desribes as "Operation Mode:Access Point".  When I switch off the cabled internet the LTE connection does not start. Also all fields for SIM card in settings are greyed. What should I do to have the LTE switched on in case of  failure of the existing cable internet. By the way, it was reason why I bought that model- the seller and its website informed that such setting is possible. Please help :)