There is nothing to cancel: the subscription does not renew itself. We do not store payment details and we take no money without you; if you do not pay for the next term, the subscription simply ends on its date.
Days you have paid for are not lost: access runs to the end of the term and you can use the VPN for all of it.
The subscription link stays in your account, but working servers are no longer served through it. The app receives a limited service profile and a message saying access has ended, which is not a client error. To stop the app connecting for nothing, delete the subscription profile in it.
The @obfsnbot bot sends reminders before the term runs out. If you do not want them, you can mute the chat with the bot in Telegram.
If you share the VPN and stop paying, access for everyone you share with ends along with your subscription. You can disconnect them ahead of time on the sharing page. If someone shares the VPN with you and you no longer want it, you can disconnect there too.
Your account and subscription link are kept: pay for any term on the payment page, refresh the profile in the app, and access works again. There is no need to import the link or set the client up again unless you deleted the profile.
The trial is not issued twice; it is meant to be used once per account. Free days are still available for inviting friends and for tasks.
Sometimes the problem is not the service but one location or one app. Look at server load, uptime and latency on the status page and try another location, or another client from the setup guides. That is often enough to make the connection steady again.