Seems like you may just need to improve mechanically, and get better at your matchups. Your Brand still has a win rate of 61%, so I don't believe that's what's holding you back. What I think the main issue may be is the other champions that you're playing. In the last 20 games you've played 8 different champions across 4 out of the 5 roles in the game, and in the last 40 games it expands out even more to 13 different champions in 4 out of the 5 roles.
**Keep playing Mid primary :: **
What I would recommend is that you continue playing Mid primary as it's the role that has the most map pressure and control while also still allowing yourself to improve/learn at the fundamentals of the game. I think also continuing to play Brand is fine after seeing how many games that you've put in on the champion, and the win rate still being so high. You should try picking up champions with a similar play style to Brand, and you may find better success. Try Annie and Orianna as your secondary champions as they have really good zone control and punish those that ignore it. Taking the losses to learn how to play their kits and matchups will be really good for you. Assassins are boom or bust champions and are not something I would recommend for you as secondary champions to play. Brand is a good zoning champion and a team fight mage. Assassins are neither of those things, and they require a completely different skill set to perform well on.
**Try Top maybe? ::
**
The secondary role that I normally recommend people play on for improving is actually the one that you haven't touched at all; Top. The reason why is because it will show you just how strong wave manipulation can be. It's both the easiest place to see it's effects on a laners, and for learning how to execute it. Champions I would recommend for top would be Darius, Sion, and Vlad. Vlad is still a ranged champion so you have the chance of having a Ranged v Melee advantage, and can heal up in lane, if you make mistakes. If you get good at Vlad you can even start flexing him into Mid. Sion is a tanky melee champ with good zone control and CC in teamfights. Good for learning to play melee champions when you're used to ranged.
If you're not against it maybe you could try to get acclimated with the role of top via a new account before you start playing it in ranked on your main. Starting a new account with the changes that they've made to the tutorial isn't as terrible as it used to be. You can even get Darius as your starting champion for free.
Dodge if you don't get a role that you're comfortable with. Being a one-tick with Brand can allow you to play support if that's where you happen to get auto-filled, which is better than most players who main something else, but playing other roles and/or champions that you don't know is bad, and taking the LP+MMR isn't going to do you any favours. Yes, dodging will take some of your LP, however your MMR will stay the same. When you've got the solo-lane knowledge down and the pool of champions is successfully expanded past just Brand then you might be able to adapt that knowledge and be competent in other roles, but I don't recommend looking to do that until you're near mid-diamond at the very least.
Right now I suggest stick to trying to play Mid only with the champions that I recommended, and if you can't get mid try to get support with Brand or Soraka. Dodging otherwise is better for your MMR than taking losses on champions/roles that you don't know well. Do that until you're ready to start playing top. If this is actually your first season playing (basing the assumtion on your account's history on OP.GG) then you're doing really good by being plat already, and I think it would be good to focus on improving gameplay even further rather than focusing on climbing.
I type too much, sorry. You can add me in-client if you'd like and we can chat, or if you'd rather not then we can talk here in this thread. Take care, and good luck, friend.