While unlikely, it's possible that the Nexus 7 doesn't support your local LTE network. It seems like Aero2 only operates LTE on 900MHz and 2500MHz (Take that with a grain of salt though; it may be incorrect or incomplete).
The Nexus 7 2013 apparently doesn't support either of those frequencies. Oddly enough, apparently neither does the iPad Mini.
You may want to contact Aero2 or whatever local providers and find out which frequencies (or "bands") they operate in your area. It doesn't matter if their network or SIM cards are "100% certified for the Nexus 7", they may not operate the appropriate frequencies in your area specifically. Rather than ask "Do you support the N7?", ask them "What LTE bands are operational in the area with the postal code [your postal code]?"
If your area should have a band that's listed on the GSMArena page for the Nexus 7, then you may have problems with your device. If not, then you may want to find a carrier that supports LTE on the appropriate bands in your area.