I would suggest that you look at the panel and locate a relay that is a exact copy of the start relay that you know is good and swap their positions . I would check all hots and ground cables ,load test battery and check starter motor also if relay is not problem.
The relays are mounted close to the starter, 1 for starter 1 for glow plugs, they are relays like you will find on lawn tractors & z turn starter relays
