Change from cable router to 5g/lte

I have had so much difficutly and frustration with O2. 

After waiting 3 weeks for the “fastest” shipping, my router arrived. I tried to set it up and checked with my landlady who ended up telling me that a cable connected router doesn’t event work with my apartment. I need to get a 5G/LTE wifi setup. Now I need to return and cancel the Fritzbox that was sent to me and order the correct 5G home wifi setup. Not even going to go into my concerns about getting properly refunded for ordering the incorrect router yet.

Changing my O2 tariff has actually been impossible.

I get stuck on the shopping cart page when I try to move forward to pay for the new setup. I have tried on my phone and on my laptop. Neither work. 

Next I tried to login into my O2 to change the “tarif” and select the 5G option. The website has a seizure and gets frozen in a loading state. What is going on?? I’ve never experienced so much difficulty trying to set up a simple wifi connection at home. This is absolutely ridiculous. 

I have limited German speaking skills as I have just moved here and there customer service on the phone has been completely in German.

Customer service at O2, please please please help me out of this situation.