بسم الله الرحمن الرحيم
The room on first floor which is built above the hall of wudhukhana and instinjakhana if its veranda does not come under the boundary of Shariah mosque and there is no threat to the mosque due to mobile tower then it is lawful to install mobile tower. And the rent which is paid by the mobile company every six month is lawful and may be spent in mosque necessities because the user will be responsible for the wrong use of net or mobile not the mosque trustees. However, no long-term contract should be made with the mobile company for the installation of tower so that in cases of need in future the place may be vacated quickly. However, it is far better to have any other resource for the income of mosque instead of mobile tower.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.