Hi !
Thanks for your quick answer and the proposed solution. With your help, I finally managed to install eOS on my Axolotl 
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For me, your last link was the one to follow in order to solve the yellow triangle shown while in Fastboot (waiting for any device issue when installing FP4 → SOLVED - /e/OS Devices / Fairphone - /e/ community).
You may be of some help with this question : when I’m restarting the phone, it says that the phone is unlock. I don’t have such message on my paid FP3+. Do I need to relock it via Command Prompt and is it not possible in order to run eOS properly ? I’m not sure 
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In addition, I did not have always the same message as described in the installation guide. Any clue who / how could I forward my own experience to allow others to benefit from it ?
Thanks for the time taken 