Here we have an unfortunate typo in the 1841 British edition that shows once more the overall sloppiness in the typesetting for that edition. In this instance, “the city Teancum” was set as “thy city Teancum”. The 1849 LDS edition restored the correct determiner, the.
Summary: Maintain the definite article the in the phrase “the city Teancum”, the expected reading as well as the reading of the earliest text.