Thank you @arjanvanb for your support. I read Gateway queries & ranges and also came across The BIG and SMALL ANTENNA topic part 1 and The BIG and SMALL ANTENNA topic part 2.
Conclusion for my issue is, that there is only the solution to replace the antenna. Currently I am looking for a collinear antenna with 1,4m height, e.g. DELOCK 12504 (LoRa, Outdoor, N-Buchse 8 dBi) and to connect it by 3m CFD400 cable DELOCK 13021 (WLAN Kabel, N Stecker, RP-SMA Stecker) and SMA-REV-Adapter to the LPS8 indoor Gateway. Currently I do not want to install it as outdoor antenna because of high effort for lightning protection.
I would be happy about any comments on the selected parts and other alternatives (available in Germany). Thank you!