this kind of parcel starting with U is china post ordinary parcel. This is a slow and cheap service(most seller uses it for low value item). Airlines and Royal Mail give China Post ordinary parcel very low priority to get space and sorted. It might take 4 to 12 weeks to reach your address.

Also to save labor cost, both China Post and Royal Mail will NOT scan such cheap parcel after it was sent to China export customs. So you can not know exactly where it is. Your seller made very serious mistake. He should not use such U parcel to ship expensive item.

If your buyer protection is expiring, you should open dispute to paypal/aliexpress for lost parcel refund. Otherwise keep waiting.