It may be due to the different distance to the mobile phone provider's transmission mast, or to the capacity utilisation of the mobile phone cells. Then the location in the flat has a decisive influence. If you have to share the same mobile phone cell with many people in the city, there is often not much speed left. I would do different location tests in the flat with the help of the Netflix test site fast.com, then at different times of day, e.g. late in the evening or early in the morning. You can quickly see the technical limit of the LTE connection. It can fluctuate extremely during the course of the day. Unfortunately, external antennas can't always help. Buying a different LTE router might also help, one that supports more LTE bands. When testing the speed, try to stand next to the router and test the speed with your smartphone at the same time.