I have 5G home and I am unable to upload an MP3 file to mixcloud.com. Everytime it retries 3x but fails each time anywhere between 23 - 29% of the upload. This is device agnositic and calling tech support is more than frustrating since I keep getting the wireless guys. I need some help here please!
Take a look at https://packetlosstest.com/ (I like to use the '1080p game stream' preset since it's the most aggressive of the tests) and see if you're maybe getting some lost packets that might cause errors in transmission.
Is mixcloud the only site you're having trouble uploading things to? Are you able to stream movies etc OK? What are your speeds like? What's your modem's signal level (shows as # of bars in the admin page)