Came across this piece of info from an article regarding his trial at Hull:
The 27-year-old also had a trial with Championship side Hull City recently and is understood to have impressed during his time with the Tigers.
https://ninetyminutesonline.com/rangers-open-talks-to-sign-27-year-old-could-be-smart-addition/
Impressed but didn't do quite enough? Or got offered a contract but reconsidered due to the amount of game time he was told he'll get?
Hopefully something positive comes out of the Rangers trial.
EDIT:
It sounds like he might be doing enough to win a contract. I'm sure we'll get an answer one way or another in the next few days.
But in today’s conference Gerrard only intimated interest in Polster and one other player. This time New Zealand international goalkeeper Stefan Marinovic. Marinovic joined the club on trial a few days ago and now it looks like Rangers will be offering him a deal.
https://www.rangersnews.uk/news/rangers-deal-for-international-imminent-one-of-two-final-signings/