Look on YouTube see if there’s a Merlin RO system setup more or less the same as.
If you google it it will come up so you can watch it on YouTube just type in Gardiner pole systems putting together an RO system,I’ve just seen the video.
If my memory serves me the end caps where secured in place by metal u shaped 'clamps.' (Like the clamps used to remove a car radio.)These instructions are for a Champ housing. The end caps are secured slightly differently.Someone suggested using a T radiator valve screwed into the port to release it. There is a Youtube video by Mark Munro.Then it's remove and assemble a new membrane to these instructions. These instructions are for a 4040 but the 4021 is exactly the same.