Im just guessing but LTE-M1 is basically a software / protocol addition to existing LTE towers, so its possible the broadcast messages for normal LTE are the same / compatible with LTE-M1, but if the tower doesn’t support the bandwidth / protocol / etc. of the modem it probably rejects it. This may be why with an LTE-M1 compatible modem you are seeing towers show up in a scan but none allowing you to connect. Total guess on my part though.
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That may be the case, definitely look at the datasheets for the module / shield / whatever you are using. Also note that 2G requires up to a 2A pulse when sending data, make sure your power supply can support that otherwise you may get dropped data, modem restarts, or other undefined behavior. Also be careful with the logic reference voltage of the chips you are using (2.8V for SIM800L) vs the power going into them and the logic voltage of the arduino (I think 5V, maybe 3.3V). Maybe your adapter board / shield accounts for this but that is another thing to double check, if you are sending serial commands at 5V from the arduino to the SIM800L you may damage / break it. If you are using shields its very likely they address this for you.
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good to hear
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For bit-rate, most of the modems from SIMCOM and other manufacturers default / play nicest at 115200 baud. I would recommend baselining that unless you have other reasons (like if you are using software serial which you should avoid anyway).