The maharzav in his commentary to Shmos Rabbah 51:2 addresses this contradiction and comes to the conclusion that there were indeed two Tents of meeting: one situated in the center and one outside. The one in the center was the main אוהל מועד where the Ark was stored and was used for all the cultic purposes, while the outside tent was only used for Torah learning (similar to the function of a modern Beis Medrash) and possibly for oracular purposes as well. This is supported by the Midrash in Pekudai (Shmos Rabbah 51:2) and another Midrash brought in the Yalkut where it is clearly stated that two tents functioned simultaneously in the Midbar. According to the Midrah the Israelites gave too much money for the construction of the Mishkan, when Betzalel asked Moshe what to do with the surplus he dedicated it for another Mishkan (which refers to the outside tent according to the Maharzav). He also supports his thesis with the story of Moses, after the sin of the golden calf, erecting a tent outside the camp and calling it אוהל מועד related in Shmos 33,
ז) וּמשֶׁה יִקַּח אֶת הָאֹהֶל וְנָטָה לוֹ מִחוּץ לַמַּחֲנֶה הַרְחֵק
מִן הַמַּחֲנֶה וְקָרָא לוֹ אֹהֶל מוֹעֵד וְהָיָה כָּל מְבַקֵּשׁ יְדֹוָד
יֵצֵא אֶל אֹהֶל מוֹעֵד אֲשֶׁר מִחוּץ לַמַּחֲנֶה
Though it was only used for a short period until the Mishkan was erected (see rashi Shmos 33:11), since it was used for the 'מבקשי ה - for those seeking Torah knowledge, they had to erect a new tent (after Moshe dismantled his own tent) so that the 'מבקשי ה can assemble and learn Torah regularly. According to this explanation, the אוהל מועד mentioned here is the one that was used for Torah learning and for oracular purposes (which was indeed the case here in 11:26).
However, it is likely that Moshe never dismantled his tent but left it outside the camp all forty years. If such an interpretation is warranted, we can say that the tent mentioned in Bamidbar 11:26 is indeed this same tent which was also called tent of meeting. According to this explanation the אוהל מועד mentioned here is referring to a different kind of tent mentioned in Shmos as the tent of Moses, and we don't have to invent other tents that were not recorded in the Torah. see also Jacob Milgrom, The JPS Torah Commentary: Numbers, p. 386 for fuller discussion.