Cristian, to solve your problem follow these steps:
1) in your survey tab, create a hidden field named ALL (with null type in ESRI type column) and put "ALL" in the calculation column;
2) in the select_one_external field, put [operators=${ALL}] (without brackets) in the CHOICE_FILTER column;
3) in the external_choices tab, put ALL for all lines of "operator" list_name in the "operator" column.
Save your XLS and test.
The case: The "select_one_external" field type work with external_choice tab only if you set the CHOICE_FILTER. If you don't set the CHOICE_FILTER, Survey123 will interpret your list as an internal list (CHOICE tab).
I hope my tip can help you. Good luck.
Adilson Furlani