What is now the frequency? Is this the "Fc" number?
So in this case; 1869,9 MHz, (or KHz, or GHz?)?
Yes, Fc looks like
Frequency
channel; since the fancy 7-segment digits indicate the band and Band 3 is indeed the 1800 MHz one (1.8 GHz).
Also one of the small dials below (indicating neigboring cells) says Band 1 which is 2100 MHz, so also a match.
On my phone the Fc field is missing, but I guess it depends on what information the Android version reveals to the app.
So, does the 1800 (CH3) LTE FDD means this device does cover the 1869,9 frequency?
Yes, it does.
What does "FDD" and "TDD" stands for?
Frequency-
Divided
Duplex uses a different frequency for network->client and client->network direction, which means these transmissions can take place in parallel and there is no need to coordinate them in time; this is how traditional radio systems, both analog and time-division-multiplex, usually work as they are designed for symmetric traffic volumes;
Time-
Divided
Duplex means that the same frequency is used for both direction in turns (this is how WiFi usually works). Since a typical mobile phone user uses much more download than upload, TDD is somewhat more efficient in bandwidth use for this kind of application because a given amount of spectrum, e.g. 20 MHz, can be distributed between the two directions in different ratio, whilst for FDD, the same amount of spectrum is always shared 1:1. But for TDD, the time synchronisation of the clients to the network must be very precise to minimize the guard times needed for direction reversal.
Nothing you could affect as a user.
So now to your actual issue, since it doesn't seem to be a band support issue, what exactly does
/interface lte monitor info 0 show on the LtAP mini, esp. the field
access-technology?