The Fritzbox 7.50 firmware brings simple Wireguard VPN connectivity. It's more secure than the old style IPSec XAuth VPN, which Android marks as "Insecure", and WireGuard tends to run a pretty lean, fast VPN.

Wireguard VPN is simple to set up, here's the instructions - ignore most of the guff on that page, skip to "setting up a vpn connection".

Here's a copy of the important part of that page, with some changes by me because it doesn't match what I saw in the UI.

 

1. Click "Internet" in the FRITZ!Box user interface.
2. Click on "Permit Access" in the "Internet" menu.
3. Click the "VPN (WireGuard)" tab.
4. Click the "Add Connection" button.
5. Click "Simplified Setup" and then "Next".
6. Enter a name for the VPN connection (for example Anna Notebook).
7. Click "Finish".
8. If asked to do so, on the FRITZ!Box confirm that the procedure may be executed and click "OK" to complete the procedure.
9. Click "Download Settings" and download the file with the extension ".conf" to the computer.

If you want the connection on your phone, open the WireGuard app and take a photo of the QR code. On Android you can then go to the top pull-down menu, add Wireguard, and every time you toggle that it will turn on the last Wireguard VPN used.