You may make sure SIM card is working when you plug it into your phone in Germany.

There is no compatibility list in Hungary to check. Or you may check that list by yourself. https://www.tp-link.com/en/support/3g-comp-list.html?model=TL-MR3420 It's better to get a modem listed to...

Do you mean Archer MR200(EU)_V2_180117 can caused unstable wireless connection? And you downgrade to Archer MR200(EU)_V2_170927, it will work well?

What's the OS of your computer? Have you disabled firewall and anti-virus? You can try to disable them and try again. Besides, you may also try to install the utility on other computers.

Is your PLC TL-WPA4220KIT? This product can only support 2.4GHz but no 5GHz. If you want to login to the web UI of it, try to unplug your main router and just connect your computer to extender. good...

you mean even there is no power LED? Is there a reset button? or try to reset it with utility. Then, try to readd it into the powerline network with utility. https://www.tp-link.com/us/faq-1615.html