Cot training

I have two kids (2.5 years and 9 months respectively). My toddler sleeps like a dream in his own bedroom but my 9 month olds sleep has been challenging. My baby has hated sleeping in his bassinet (now changed to a cot) since birth. There was one point where I thought I nailed cot training but then he got sick and only wanted to sleep in my bed with me. He is now well and I’ve been trying to cot train all over again. Does anyone have any tips? In the cot, he wakes every 30-45 minutes (he can’t seem to connect sleep cycles), but when we were co-sleeping he would sleep through the night/wake once for a feed. I really want him to be able to sleep in his cot and any advice will be appreciated. Thanks everyone xx