Hi!
Sorry for my bad English, I'm from Kazakhstan :)
I'm also sorry for the nub question, I'm new, and this is the first time I'm dealing with TP-Link, and especially with the ER7212PC model.
A little background information:
We purchased the TP-Link ER7212PC model, I added a couple of candles and a couple of WiFi points to it. I've set it all up. Great. But I wanted more - I wanted a guest portal for guest WIFI. I read the manuals, it says that it is possible, but there is no menu item. I asked ChatGPT, he said that maybe I have an old firmware, and I need to update. I started looking for the firmware for my model - the search returned that the latest ER7212PC(UN)_V2.6_2.1.2 Build 20250705. I downloaded it, tried to install it via Manual Upgrade, specified it, pressed Upgrade, the wheel with the inscription starts to turn:
Are you sure you want to upgrade the integrated router? To avoid damage, please do not turn off the device during upgrading. And then in a circle - again the inscription Upgrade or Cancel. And don't update. 
At that time, I had:
Controller Version 5.8.22
Model ER7212PC 1.0
Firmware Version 1.0.3 Build 20230314 Rel.21018
I found this firmware: Firmware Version: 1.3.0 Build 20250117
She got stitched up. I thought that after that I would be able to install the latest version from the ER7212PC(UN)_V2.6_2.1.2 Build 20250705 website. But no. Now, when selecting the firmware, an inscription appears: The maximum file size has been exceeded. Maximum allowed size: 128 MB.
What am I doing wrong? And how to install the latest firmware? Or was this Firmware Version: 1.3.0 Build 20250117 the last one?
And whether the firmware will solve it (if possible) my problem with creating a WIFI guest portal.
Oh, sorry for the long text, but we Kazakhs can