Hi Jef, thank you for using our forum, my name is Luis I am part of the Small business Support community. I apologize for your inconvenience you are having Jef, could you please, connect a computer directly to you modem device and run a speedtest to verify if the speed is correct?
You can try to reduce you MTU size to 1450 and try with different sizes to find the right one.
I hope you find this answer useful
Greetings,
Luis Arias.
Cisco Network Support Engineer.