I have a SIM that configure to work on Download speed of 7Mb
but when I connect it to the MC8790V modem I only get speed of ~ 3Mb
when I use another modem MC7710\ZTE I get ~7Mb at download
do I need to change something in the AT command?
MC8790 is a rather old HSPA module and has a peak download of 7.2Mbps. You will only get close to 7Mbps in perfect conditions if ever. 3-4Mbps is rather normal for a HSPA module.
MC7710 is a LTE module that support 100Mbps downlink so with that module it should be easy to reach 7Mbps.