Some questions about NR7102

Hello! I have purchased a brand new NR7102 router. Here are some questions I would like to have insight if possible.

Zyxel Air app speedtest issue: I have tried using the Zyxel Air app but it doesn't help me much in installing the router properly. The in-app speedtest is totally different than the speedtest done through 802.3. Why is that and how are we going to know that the current fixed position of the router on-site is optimal for maximum speeds?

Access Technology Tab (in Broadband -> Cellular Band): Why CA in LTE-A mode has download/upload speeds around 3-4mbps best? In 5G NR mode the router never connects to provider. My guess is because this must be 5G in SA mode which in my country (Greece) is not yet supported. Is this correct?

Mounting question: Be advised that the router faces a tower located 861m away (at the same altitude with a crystal clear LOS). Also, RSRP values are Excellent as per the Zyxel Air app (around -74dBm and -67dBm). RSRQ values are around -8db and -13db depending on cell and SINR values are around 1dB and 10dB (at best 11dB). These should not indicate by any means a bad or medium connection. Specifically, for n78 the SINR skyrocket to 40dB and then plummet to 3dB. Is this normal?

Poor performance: Finally, the router aggregates n78, B7, B1, B7 and B3 (five CCs in total at any given time) but my mobile phone while aggregating the same CCs at the exact same location gives me double the download speed (400-500mbps with peak 600mbps on my Xiaomi 12 versus 250-300mbps with peak 400mbps for NR7102). Can you give any insight on the aforementioned?