The difference between the rj45 and the rj11 is how they are both used. The RJ45 comes with eight pins while the RJ11 only comes with six pins. The RJ45 is the type of cable that is normally used when you are using an ethernet cable while the RJ11 is commonly used to connect your landline.
When you would check the size of the two, the RJ45 is known to have a bigger size as compared to the RJ11. You are not allowed to plug in the RJ45 to the port of the RJ11, but you can use the RJ11 for the RJ45. This might be because of the size difference between the two. Still, you should choose the correct ports to avoid possible issues in the long run.