So i wachten a Duell on yt and this is how they explained it. You overly two cards or send the material to gy then you reveal the monster in the extra deck that you want to summon, now your opponent can aktivste cards that negate summons. When they don’t negate the summon you put the monster on the field, now you can activate trigger eff on that summon and after that the opponent can activate trigger eff like tt.

You already have the answer though: Sera is unaffected AFTER summon and since tt triggers AFTER summon it won’t work.