Жестко закодируйте список всех лет и месяцев и оставьте соединение с существующей таблицей:
with ym as(
select * from (values ('201401'), ('201402'), ('201403'), ('201404'), ('201405'), ('201406'), ('201407'), ('201408'), ('201409'), ('201410'), ('201411'), ('201412'),
('201501'), ('201502'), ('201503'), ('201504'), ('201505'), ('201506'), ('201507'), ('201508'), ('201509'), ('201510'), ('201511'), ('201512'),
('201601'), ('201602'), ('201603'), ('201604'), ('201605'), ('201606'), ('201607'), ('201608'), ('201609'), ('201610'), ('201611'), ('201612'),
('201701'), ('201702'), ('201703'), ('201704'), ('201705'), ('201706'), ('201707'), ('201708'), ('201709'), ('201710'), ('201711'), ('201712'),
('201801'), ('201802'), ('201803'), ('201804'), ('201805'), ('201806'), ('201807'), ('201808'), ('201809'), ('201810'), ('201811'), ('201812'),
('201901'), ('201902'), ('201903'), ('201904'), ('201905'), ('201906'), ('201907'), ('201908'), ('201909'), ('201910'), ('201911'), ('201912')) a(YearMonth))
select ym.YearMonth, at.AccountNumber, mt.*
from ym
cross join [Accounts Table] at
left join [My query of transactions with YearMonth and Accounts] mt on mt.YearMonth = ym.YearMonth and mt.AccuntNumber = mt.AccountNumber