Hello@CreNet ,

Welcome to our community.

If you are experiencing slow Internet speed with your TP-Link 4/5G LTE Router working in 3G 4G Router Mode, here are some steps to help troubleshoot the issue:
1. Ensure the router is located in an open area, preferably near a window. Obstacles can hinder the signal. Ensure the signal indicator/strength on the router gets at least 2 bars (75% or higher).
2. Ensure the router's firmware is updated to the latest version. 
3. Assess Device Connectivity:  
   1) If only wireless devices are experiencing slow speeds:
      Please log into router's web interface and navigate to Advanced > Wireless > Wireless Settings to adjust the Wi-Fi channel and channel width. 
   2) If both wired and wireless devices are slow:
      A. Go to Advanced -> Network -> Internet page and set Network Mode on "4G Only". 
      B. Contact the ISP to get another available APN and manually create a new APN profile. 
      C. Try to select different 4G band(s) manually. 

For detailed guidance, please refer to: How to troubleshoot if I have low speed with TP-Link Wireless 4G LTE Router working as 3G 4G Router Mode
 

Note: You might use a mobile phone to compare the speed, however, please kindly note that most mobile phones now support much higher LTE categories such as Cat19 and Cat20, or even 5G, the higher the LTE category supported, the higher the maximum speed it could reach, so it doesn’t make sense to compare two devices speed with different LTE categories supported.

Best Regards.