All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If you assumed Ihraam at the Meeqaat determined by the Sharee‘ah and maintained Ihraam until you performed ‘Umrah, then what you did was correct and your ‘Umrah was valid. It would not have permissible for you to assume Ihraam at At-Tan‘eem because you were already in the state of Ihraam.
That ‘Umrah, however, took place outside of Ramadan because one of its pillars, namely the Ihraam, took place before the month of Ramadan started. Assuming Ihraam at At-Tan‘eem or Al-Ji‘irraanah is permissible for those who are already in the sanctuary and out of the state of Ihraam. If they want to make ‘Umrah, they need to go outside the sanctuary to assume Ihraam. Otherwise, they must slaughter a sacrificial animal because they did not combine the two states of being inside and outside the sanctuary during the Ihraam for the ‘Umrah. If you want to make the ‘Umrah within the month of Ramadan, then you must go outside the sanctuary - and At-Tan‘eem is considered outside the sanctuary - so as to assume Ihraam, and then perform ‘Umrah.
Allaah Knows best.