I have a theory.
Ultraman Trigger might not be a direct sequel to 1996 Ultraman Tiga TV Series & might not be taking place in the same universe as Neo Frontier universe we know.
The story guide given by Tsuburaya Productions doesn't mention about humanities encounter with Giant of Light - Tiga & Dyna in 2007-2010 & 2017-2020. It directly points to the 30 million years old great war where Ultraman Tiga had fought the Three Dark Giants
It's like there is a Parallel Universe where Tiga does exist. But the events of 1996 TV Series never happened in this universe. We have already seen there was another Ultraman Tiga who had existed in the Ultraman X universe. Trigger's Universe might be similar to that.
Tsuburaya Productions also claims this series to be an homage of Ultraman Tiga and never mentioned anything about sequel.