Slow to connect and then connection slow or dropped

I had a 75/75 old internet connection working fine. Then, a technician came and updated the ONT box and the modem to the 1G service. Since then, it takes a very long time for any device to negotiate a connection with the router and, once connected, it works fine for a while and suddenly starts to work slow or just drops the connection (all the browsers return a message saying there is no internet connection). 
I have an older Airport router connected to it and that one always works well. Thus, it seems the problem would be the wifi of the new router. I tried doing a factory reset and rebooting it several times, but did not help at all. I turned on and off the 6G band but did not help. Still, when a new device wants to connect, it takes several minutes for it to start working and then, at random intervals, it drops the connection or goes very slow. At that point I have to connect to the Airport router to keep working.

Any ideas of what might be going on? 

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