Could you perform a speed test using ethernet (wired)? If this is not possible, could you perform the test with the device in close proximity to the SmartWifi modem?
In the previous topic you mentioned that the meter cupboard is located outside of the apartment, which may cause insufficient Wi-Fi signal strength.
If the speeds remain extremely low on ethernet and/or a (good) Wi-Fi connection, the problem might be caused by the router's internet connection. Since you are a new customer, the router might have a Dual-Stack Lite WAN configuration assigned to it, which exhibited speed and stability issues in the past. You may perform a Google search for 'my ip address' and note the first half of the address in this topic, which will allow me to determine whether DS-Lite is in use.
What are the approximate maximum download and upload speeds that you are able to achieve?
The downstream/upstream data i.e. power and MER levels are good to excellent.