[Q] SIM Toolkit missing and 2100 MHz problem on 3G topic

I don't seem to find the SIM Toolkit even in the application list from settings. I need this because the tablet doesn't work with a SIM card from Digi Mobil Romania which only uses this band. I called at their service and told me to access the sim Toolkit app because there is an option 'Change IMSI' which allows me to connect to a national roaming network(crom Vodafone) which uses the 900 MHz band.

Plecase help me If You can. I need to use the sim only for internet(actually unlimited) but cannot connect to their network. IT remains on 'No Service' status even If IT says that it is registered in the network.

I need to mention that I am not an advanced user but I know some stuff.

I need to fix one of those problems(best would Be If both would get solved). I mean getting a successful way to install an stk.apk or to solve the problem with the 2100 MHz network on 3G.

P.S. My Nexus 7 2013 is LTE, rooted and unlocked its bootloader(even If I can't access its recovery menu- says no command), still not upgraded to Lollipop( my version is shown in picture)

Thank You in advance! :)