R4 3DS card not work,how to fix

I got a R4 for my 3DS before. It works well on my 3DS console. But I upgrade my 3DS to V2.1.0-4U by mistake, and the R4 3DS card can’t work any more. Just show: “error” on the screen. What can I do to fix the problem? Or should I buy a new R4 3DS card?
answer 1:
You’ve made a huge mistake by updating. The R4 is an unacceptable device unlike a disease towards consoles and companies that actually want to make money off their products.r4i 3ds card , But I’m not here to judge you, I have an R4 chip myself. In the simplest way I can think of explaining, your files that keep the R4 running are out of date. By updating, Nintendo made the 3DS able to reject the current R4 software. If I were you, I’d try to contact the company or (if possible) get the newest files to run the R4 that find their way around the new update. For all we know, it may not be out for a while. What you have experienced isn’t a problem with the 3DS or the R4, it’s basically like putting a Wii disc into a PS3. It won’t read the disc.
answer 2:
I think your r4 3ds card is the old version. You need to download the latest firmware from official website, and upgrade your card. Currenly, many r4 3ds cards can work on 3DS V2.1.0-4U, I think you card also. Try to upgrade the card.
Try to upgrade your R4 3DS card.
You need to download upgrade patch and newest kernel,and then upgrade it in DSi(Lower than v1.43) or DS Lite.
answer 3:
No need to buy a new one now, just need to update your r4 3ds card through download and install the latest kernel, then you can fix the problem.So try to find right version kernel from the official website, before install suggest you to format your sd card first.