If you think your system is not functioning properly the first set would be to open the modems SCC at 192.168.0.1
The color of the icons marked as 1 & 2 will give you a quick peek at the systems condition including FAP status and any active error conditions.
If not in FAP then the next step would be to connect a single computer directly to the modem and run a series of speed tests at testmy.net.
Create an account there, log in and run 3 to 5 speed tests spaced at least 5 minutes apart.
Use the 12MB manual test.
post the link to your saved results here in the Community.
Are you by any chance connecting via a VPN ?
When dealing with a data capped service you have to be VERY careful with what is allowed to connect to your network.
It is also necessary that wireless devices in particular are left in a true powered off state rather than in hibernation when not in active use.
As to speeds, the more devices you have connected at any one time, the slower each will appear to be. They are after all "sharing" a rather small data pipe.
An "effective fix" starts with collecting information and troubleshooting.
That is going to entail determining what your systems baseline performance levels are with a single computer connected directly to the modem and then adding elements of the network back in a controlled fashion.
"There are time I have been the only one connected, and this is just my laptop and internet access"
Again I ask VPN ?
"NOT downloading anything"
That you are aware of. Have you installed Glasswire on that computer so that you can see usage of not only the programs that you can see but also background processes that you normally would not be aware of?
" I still have awful service and it goes totally out"
Please define "totally out".
How many LED's are lit on the front of the modem when this happens ?
What error codes are present in the Modems SCC when this occurs ?
What is shown in the Modems State Code Monitor ?
"We do not download movies or documents, so we should be good for the plan we have; but that is not the case."
Pure supposition. Are you in FAP or not ?
If not, have you ran speed tests as I outlined above to see what the actual speeds are under the defined circumstances?