Look at the sticker on the bottom of your router. It must say Ver: 5.x . Installing firmware meant for V1, V2, or V3 will likely "brick" (permanently disable) your device.
Before downloading any firmware, confirm you have the v5 model. tplink mr3420 v5 firmware
Ultimate Guide to TP-Link TL-MR3420 V5 Firmware: Updates, Features, and Optimization Look at the sticker on the bottom of your router
If you want, I can:
If the update page doesn't load, clear your browser cache or use a different browser. and Optimization If you want