There could be any number of things that might constitute "poor cloud connectivity" so instead of pinging you with a bunch of questions (which might go slowly over an online forum) it might be fastest to open a case with Meraki Support. They will be able to see if and how your MS350 switches are communicating with Dashboard. Are the M350 devices physically stacked, and if so, there is a specific sequence of steps, did you follow the correct procedure to create that stack? Are the switches showing up in Dashboard at all with a green or amber status, or only as red offline?
It could be an upstream firewall issue for example, if you have a restrictive Internet firewall in the outbound direction, you'll need to make sure a handful of ports are allowed out (such as UDP/7351 for primary Dashboard communications) and you can see the full list in Dashboard by going to Help > Firewall Info. If you are behind a web proxy, you may need to leverage the local status page via the MS350 management port to put in a proxy configuration. In fact, you can connect into the management port and perhaps see if the local status page offers a clue about why it doesn't have full connectivity to Dashboard.
If you're brand new to this, and since it could be many things, might be fastest to work with Meraki Support, but let us know if we can also help.