The centre-half, who is expected to feature for his new club in their first home pre-season match against Nottingham Forest on Friday evening, has had stints at Welling United and AFC Wimbledon.
He also made 30 appearances for Luton Town following an 18-month spell with the Hatters before he was released earlier this week.
“I am really pleased to get it all sorted,” Franks, 24, said of his switch to the Sky Bet League 2 outfit, now managed by former Manchester United and Tottenham striker Teddy Sheringham.
“It is a new challenge for me and chance to prove myself and I am really looking forward to getting going.”
Speaking to the club’s website, Franks added: “Teddy contacted me a few weeks ago and after a couple of conversations I thought this was the right club to choose and I am glad it has worked out.”