I did FM transmission and it fades away just in another / next room .... I thinks it is an issue with low power transmission....
I have standard alloy steel telescopic antenna.
That's the problem of your antenna and walls. Your walls working like shielding.
And these Chinese "telescopic antenna" are very bad antennas, because they are not matched with 50 Ω connector. They are intended for receiver (for which matching is not so critical), but they are not intended to work with transmitter.
For transmitter you're needs properly tuned antenna with matching between transmission line and antenna. Otherwise it won't accept the power from transmitter.
Another question - does limeSDR mini has an option to see what is it its power output at different let's say frequencies or modulations? Just to know how many dbm or mW is delivered into antenna and from antenna into air ?
You're needs to measure it. Here is power output for different frequency:
http://www.dd1us.de/Downloads/LimeSDR%20mini%20TX%20output%20power%20in%20the%20VHF-UHF-SHF%20bands%20rev1.pdfSince you're using just a metal stick (with no tune and no matching) instead of antenna, I bet that very small part of transmitter output power is delivered to antenna, because your metal stick is not matched with 50 Ω transmission line and your metal stick doesn't have counterpoise. Without counterpoise it's impedance will be very high, so it has very high VSWR and power is just reflected back to the transmitter.
You're needs proper antenna, properly tuned and matched.