When I did a search in my Google Drive folder of 4e era Dragon and Dungeon magazines, i got these hits on the word “Tethyr”. Now that could just mean the country was mentioned in one sentence; I’m a bit too lazy to crack open each PDF. But hope this helps.
Dungeon
170 (somewhere in the entire issue, before they were split into individual article PDFs)
183 (ditto)
190 EOTR - The Wild Lords
191 Eyes on the Realms - Jalander’s Dodge
195 EOTR - Spellslayer Wine
196 EOTR - Ghost Knight of Galardoun
198 Cormyr Royale
206 “Eyes on the Ball” adventure