Note you probably have the older v1 version. The one I found was a v3 hub which uses a the smaller USB connector than the V1 and only needs to plug into one USB port on a hub for power.
You just need to follow these instructions after reading the rest of this..
One vital bit is missing. Make sure switch, next to Ethernet jack is set to AP mode. In other modes the MR3020 is hard coded to come up as 192.168.0.2 and wants to be your router which might cause issues with your network.
If you need to get TFTP running you should be able to google plenty of quick how-tos like this one for CentOS 7.
Be sure and test TFTP is working from another PC like
Verbose mode on.
tftp> get tp_recovery.bin
getting from 192.168.0.225:tp_recovery.bin to tp_recovery.bin [netascii]
Received 8126464 bytes in -5.9 seconds [-11040379 bit/s]
Note if you do not have a Linux PC to try it from get Cygwin
Now what you might not have caught above is the IP address have to be 192.168.0.225. If you have only a network with addresses in of 192.168.0.X then you should be good. If your network is a more common address like 192.168.1.X then you should still be OK with the ip addr add dev eth0 192.168.0.225/24 instruction to fake the address. Problems arise if you have multiple networks, like you should be doing, and one of them has addresses of 192.168.0.X. In that case both the TFTP server and the hub being flashed need to be on that network.
Now holding the button 15 seconds seems like it might be too long or too short. You want to hold it till the second LED starts blinking to trigger the download of the software. I discovered this by trial and error while watching traffic to the TFTP server port.
All that said when I tried this my network seemed to get royally screwed up. Many of the things on the 192.168.0.X network needed repowered, not just rebooted, to get them back online. Might have been coincidence but happened just as the hub came back up. The flash did seem to work though.
That just left the problem of the MAC having changed. Even though you can spoof the MAC in the underlying OpenWrt code (which might fool your router) the Meteobridge software pulls the MAC directly from the Ethernet card so the Meteobridge license and Ambient Weather Network uploader sees it as a new station. Meteobridge says to email them at info(at)meteobridge.com with the old and new MACs to transfer your license in the 14 day trial period. I'll have to update whether that works. Ambient Weather Network on the other hand I seem to have to add as a new station. That seems to work on their end but when I tell the Meteobridge to upload to it I get
Meteobridge settings successfully reloaded
Error: Ambient Weather Network: EXPIRED
Still trying to sort that.