You may be right that your flip phone may not be able to connect with a camera so that you can watch your mini. I'm thinking that your tablet may have bluetooth, so that would be the way to go. I've never heard of a camera subscription that you have to pay for monthly. I bought my cameras outright, about $125 a piece and have had them over 5 years, so they have more than paid for themselves. I'm sorry that you're scared to change out your router, but I can understand it, if you feel that every time you touch something, you have to replace something else. Perhaps you know someone that could help you navigate with the research, purchase and installation of a camera. Teenagers or young adults are great with this stuff. I think my grandchildren know way more about this tech stuff than I do. What I know, I did look up on the internet and then when I had to install things, I watched a YouTube on it.
Years ago, when I was going on a 5 week vacation, I was freaking out about not being able to see my horses to make sure that they were ok. My farrier, who raises percherons, told me about webcams and I've had them ever since. Maybe you know someone else that has cameras in their barn or on their property that could give you a hand. It's hard to help you from Connecticut, though, I wish I could.