Hello,
I have just gotten Minecraft and I have tried to play with a custom skin and it didn't show up in the single world player or the multiplayer in my server. I already checked if I had any malware by using Malwarebytes scans and checking if I have any incompatible software installed, but there was none. I don't seem to have the folder .minecraft/assets/skins. I also tried to delete and redownload Minecraft several times with no change and I do not have the Premier Opinion application. I cannot check the routers though because of personal reasons. I have the problem of trying to join servers, but unable to, with the error message "failed to login: the authentication servers are currently down for maintenance." My relative was able to play in the servers while I was unable to play while they did for hours. They logged in and out, but they could still play, so we were confused why I was unable to. I am sure I am not using incompatible or illegal software and I do not have firewalls blocking Minecraft or Java. The solution in MC-207896 did not help me and I have tried the instructions step by step twice.