@Santatizer keep trying with the dummy. As PP poster says, non-dummy users need significantly more parental input to sleep without crying at this age. You're on the cusp of If you don't establish it now you never will, because after 5 months old it's much harder to properly establish.
Say you don't though - then the answer is to (a) tolerate your baby crying more when going to sleep. Or (b) Add in lots of extra parental help.
(a) would involve staying with your baby, not leaving baby to cry. You would put baby down in the cot fully awake, then pat and shush until asleep, all done in the cot. This is going to involve a lot of crying if you don't establish a dummy for comfort sucking. So don't assume that crying means it isn't working. Crying is inevitable and you should be expecting it and emotionally prepared for it yourself.
(b) Would involve things like feeding to sleep and leaving baby on breast/in arms, rocking to sleep in your arms, contact naps, cosleeping, cuddling to sleep. Or pushchair movement, car ride naps, bouncer rocking. There probably will still be some crying with any of these (apart from feeding to sleep) because of nature's push for babies to comfort suck. Without it, you tend to get baby crying to some degree. So again, don't assume crying means method isn't working - crying is inevitable.
pram outside... creates and unsustainable sleep associated
I've picked this out because it exemplifies lack of understanding of baby sleep physiology. There's a lot for me to try and summarise into a few paragraphs here, but I'll try.
Your baby will need your help to feel comforted until about preschool age. Babies/toddlers that sleep independently don't do so without any comfort, they use props to help them comfort - dummy, blankies, self soothing movements. At 5 months your baby doesn't have the motor skills to even do any of those - they need help to use/do their comfort mechanism.
Unless your (very bright) 21 week old has the emotional development of a 4 year old and the fine motor skills of a 12 month old - then baby isn't physically capable of soothing without your help in some way.
The pram/bouncer is an independent way of allowing movement to comfort. By that I mean it's easier to wean off that the others, on a journey towards independant sleep. The line I quoted suggests you are expecting independant sleep right now. That's what I'm trying to explain as unreasonable.
we suspect she is...
[CRINGE]
My eldest daughter was walking at just turned 7 months. My second daughter at 9 months....
Absolutely none of that is of any relevance whatsoever to sleep. And it's offensive to suggest those who's babies sleep well are less bright.