as much as I love Mikrotik's products (have couple of them and I'm perfectly happy with them and recommend them everywhere), I just have to share my "frustration" with Chateau LTE12 (which I wanted to be a perfect replacement for my otherwise working LTE12 modem from another vendor - but I wanted the Mikrotik experience instead because of the convenience / features we all love from RouterOS).
I have literally tried every single step / tutorial / workaround I've been able to find on the web to get the external antenna work on the Chateau LTE12 just like the antenna works with my current LTE12 modem. With my current LTE12 modem and a Poynting Xpol-2-5g antenna pointed directly on top of a building to a visible cell tower in a nearby village (no obstacles whatsoever), I am able to connect to BC3@20Mhz (1800Mhz) bands and get surprisingly high download / upload speeds (on average 80/40 mbps).
I have thought that Chateau LTE12 will behave the same (if not better) - like a drop-in replacement for my current modem, but no matter how, the Chateau LTE12 will connect only to the BC20@10Mhz (800Mhz) bands and will not even see any BC3 LTE FDD band via Cell Monitor. Cell-Locks to different phy's will not help either - download speeds are extremely low (ranging from 2-5mbps), whilst upload seems sufficient (20-30mbps) - obviously there's something going on (???). The SMA pigtail connectors were sufficiently fit / secure on the Chateau backplate. Have tried several External Antenna modes (auto, both, ...), but it behaves still the same. Sometimes the External Antenna Selected field doesn't show anything.
I've been testing all this with RouterOS stable 7.1.1, modem firmware upgraded to the latest version (as told from WinBox interfaces dialog). Have not bothered to try latest testing RouterOS version (7.2rc*). Also, have not bothered to try internal antennas, as the LTE coverage signal in our village is below average for literally everyone (circa 15/15mbps) - that's why I've opted for external antenna placed on top of our house.
I still have couple of days left before I can return it for no return fee (do not want to), but if somebody has similar experience how to get this working (i.e. Chateau LTE12 + external antenna -> BC3 LTE FDD bands visible and connectable), I'd be happy to give it another try. (Or maybe the modem just doesn't like this particular antenna, who knows).