They do sell this pigtail:
https://mikrotik.com/product/acsmaufl
Which should be a hint towards the DIY path, but if I get it right the U.fl connector(s) are on the actual modem card, so it can be applied to any of their LtE products.
Example for the WAP ac LTE6:
http://forum.mikrotik.com/t/wap-ac-lte6-kit-external-antenna/139264/1
But the Chateau series do have external antenna connectors:
http://forum.mikrotik.com/t/lte-modem-with-external-lte-antenna/169758/1
those should be the pre-made “device internal/antenna external” approach you are looking for.