1) "Hatta in Arabic حتى" means "until" and is always coupled with time (not location). It is also found in Quran 97:5 ( سَلامٌ هِيَ حَتَّى مَطْلَعِ الفَجْرِ) and it always refers to time. The word that does refer to location is "haith حيث" which means "Where" and is always coupled with a location. If the verse is a location as skeptics claim then it should have been preceded by "where حيث" and the verse should have read "حيث بلغ مغرب الشمس" in that case the verse indeed would have meant the setting "place" of the sun; however this wasn't the case. The verse started with "until حتى" which only refers to time. So "setting of the sun مغرب الشمس" is time of sunset (not a location). However it was mistranslated to the "setting place of the sun". The word "place" was added by translators.

2) In Arabic grammar all places and locations are referred to in the male mode. For example, "هاذا المكان"... Since the setting place of the sun is a location then it should also be referred to in the male mode however this was not the case. This verse ends with female mode: "he found at it people وجدعندها قوما" here "it ها" the Arabic is in the female mode; which means it refers to the muddy spring (عين حمئة female mode) and not to the "setting place of the sun" (male mode).

So both "until حتى" at the beginning of the verse and the female mode at the end prove that the translation was wrong. Quran 18.86 simply says that Thu Al-Qarnain continued moving until time of sunset, while setting he (and his army) were in a muddy spring and there he found people... So the sun setting in a muddy spring is just a mistranslation.

In the Quran there are sunrise and sunset and also moonrise and moonset: