Unfortunately upgrading the sim card is not going to improve coverage. The canyon and mountains can definately cause interference and reduce signal. Signal from a tower has a radius of 7-10 miles. according to the coverage of 91301 there is veried coverage from moderate to good. Again there may be interference. You can contact our support Customer Service Chat for alternative solutions.
Thanks again for posting
BTW I graduated a Eagle from OPHS. Miss that area greatly.