How to get GSM into a reinforced concrete structure - Faraday cage?

Hello!
I face the problem of getting GSM signal into a reinforced concrete structure. It acts like a Faraday cage so there is not signal inside.

Due to legislative I can not use active repeater. So how can I get it inside passively?

Outside of the building a cellphone shows "full" signal. I thought I will buy two GSM antennas connect them with a coax cable. One antenna would be outside one inside. Will it work?

How much gain should antennas have? I know "higher is better" but give me a starting point so I can decide if it is doable.

edit: The nearest cell tower is only 2,5km.

Passive link is a good alternative if cellphone distance to slave antenna is very short.
If resulting coverage is good enough for your needs, is not possible to calculate with the data you have provided.
Can do a simple link budget with assumed values.
Signal strength outside your building: 200 uV = -60 dBm
RX antenna +10 db gain => -50 dBm
10 dB cable losses => -60 dBm
Inside omnidirectional dipole 0 dB =>-60 dBm
3 meter distance to your cellphone=> 50 dB path loss. => -110 dBm.
Phone antenna gain -4 dB => -114 dBm => Too weak signal. (assuming min -100dBm for reliable GSM signal)

Replace my numbers with your actual values.
Do same type of calculation the other direction also, start with typical phone TX level +30 dBm.

Path loss calculator: Intelligence Support Group, Inc. - Free Space Path Loss Calculator