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The L535 is limited to 3G connection only and mobile data speeds, therefore, may also be greatly limited. Could you test the bandwidth of your current mobile data connection, e.g. by using Microsoft's Network Speed Test app (try downloading this over Wi-Fi first).
Other solutions you can try are:
- restart your mobile data connection (by turning on Airplane Mode and then turning it off again)
- move to a location (near the window, avoid places with a lot of concrete) that has better signal reception
- place the upper part of your phone on top or near a large metallic object (tried this for several phones and it seems to work sometimes, not always) - the antenna of the L535 is in one of the upper corners of the phone