Welcome to the community!
If you're interested in financing a new phone, you should note that Fido has now switched from their subidised plans to a financed payment program for new devices. That is, the cost of devices is now separate from the cost of services. It's likely your old plan is a subsidised plan. If you are unable to select your existing plan, it's possible your old plan is not compatible with the new financed model. If that is the case, you would need to switch to a new plan in order to finance a new phone. On the other hand, you would be able to keep your current plan if you purchased your new phone outright.
If you would like to discuss whether your current plan is compatible with the new financing model, you would need to contact customer service as they would need access to your account. Alternatively, you might consider sending @FidoSolutions a PM. Once they verify some information, they'll also be able to access your account.
Hope this helps 😀