In many translations of the Quran Umma Al Qura in verse 6:92 is translated as the city of Mecca/Makkah. But when you read the verse carefully you will see the verse is referring to the Mother/Capital of Cities/Towns (Qura).
6:92 And this is A BOOK which We have sent down, blessed, affirming what is between his hands, and that you may warn the Mother of the Cities/Towns (Umma Al Qura) and those around it. And those who believe in the Hereafter believe in it, and it is they who are ever-mindful of their Closeness/Attachment (Salat).
The Umma Al Qura is mentioned again in verse 42:7.
42:7 And thus We have inspired to you Arabic Quran, so that you may warn the Mother of the Cities/Towns (Umma Al Qura) and all around it, and to warn about the Day of Gathering that is inevitable. A group will be in the Paradise, and a group in Hell.
So Umma Al Qura (the Mother of the towns) is not only referred to the city of Mecca/Makkah but it is referring to other Mother/Capital of different Cities/Towns. Prophet Muhammad was commanded to warn Mother/Capital of various Cities/Towns. Therefore the claim by Sectarians that Prophet Muhammad only came to warn the city of Mecca/Makkah is not true.
The root word of Umma.
ummun/umm: mother, source, principle, prototype, origin, prototype;
The root word of Qura.
Qaf-Ra-Ya = To entertain a guest, that with which a guest is entertained. To collect, run to/into. To explore; investigate; with a much and diligent search, city/town.
Finally the verse 6:19 which does not mention the city of Mecca/Makkah but instead God said to the Prophet to tell the people that I have come to warn you with the Quran and WHOMEVER IT REACHES.
6:19 Say: “Which is the greater testimony?” Say: “God is witness between me and you, and He has inspired to me this Qur’an that I may warn you with it AND WHOEVER IT REACHES, that you are bearing witness that with God are other Gods!” Say: “I do not bear witness!” Say: “He is only One God, and I am innocent of what you have set up!”