It sounds as Zigbee is probably working as designed and you are just not following best practices, or?
Because you actually have to add many Zigbee Router devices and yes you have to place them at interval, however be aware that almost all main-powered Zigbee device are Zigbee routers too, but all of them might not work as well as others do.
If you add one or more DIY Zigbee Router products (Zigbee dongle with external antenna re-flashed with Zigbee Router firmware) and place them near to your window then it can probably reach 100-meters or more as long as it is line-of-sight (because Zigbee signals have radio propagation so do not pass through dense/thick building-materials).
Did you not read and follow my guide? How many Zigbee Router devices do you have and have you placed them so they spread out? And what other actions have you taken to avoid EMI interference? Do read and try following all the tips in the guide as it is more than likely to sort out your range and coverage issues. See → https://community.home-assistant.io/t/zigbee-networks-how-to-guide-for-avoiding-interference-optimize-using-zigbee-router-devices-repeaters-extenders-to-get-a-stable-mesh-network-with-best-possible-range-and-coverage/515752