ASUS RT-AC88U poor range compared to AC68U

I just recently bought the RT-AC88U as i needed one router for my home country. I live in Middle East and we have very thick cement walls which blocks signal, and my router is placed one meter above the ground as wall mounting isn't an option because of my apartment which is rented.

I've noticed as soon as i switched to my new RT-AC88U the signals have dropped considerably, which causes speed issues in the opposite room and the room next to it.

So my thoughts on this are that the RT-AC88U...

ASUS RT-AC88U poor range compared to AC68U