@AliDE 

4G Internet speed can be affected by lots of factors, if you are experiencing low-speed issue on TP-Link 4G LTE Router, please insert the same SIM card into another 4G device with the same UE Capability, such as LTE Cat4 or LTE-Advanced Cat6 etc., then compare the speed to see if it is really the router issue.

For example, in your case, MR550 is an LTE Advanced Cat6 product that could aggregate two bands at most, while mobile phones usually support much higher categories which could aggregate multiple bands, thus usually mobile phone speed will be faster than MR550.

Once the issue is confirmed, please follow the steps below for troubleshooting:

Case 1: Both wired and wireless connections are slow on TP-Link 4G LTE Router:

    1) Go to Advanced -> Network -> Internet page and change Network Mode to "4G Only" especially when the router is working on 3G.

    2) Try another available APN profile or create a new APN profile manually.

    3) Try to select different 4G band(s) manually, refer to this FAQ: How to select 4G band manually on the web interface of LTE Gateway Routers.

    4) Upgrade the firmware to the latest version.

Case 2: Only wireless connection is slow on TP-Link 4G LTE Router:

    1) Change the Wi-Fi channel and channel width on Advanced -> Wireless -> Wireless Settings page.

    2) Upgrade the firmware to the latest version.

For more troubleshooting steps, please refer to this FAQ: How to troubleshoot if I have low speed with TP-Link Wireless 4G LTE Router working as 3G 4G Router Mode.