Fritz Box isn't bad but if you think about a new router, better watch out for routers with integrated VPN CLIENT.
And try VPN, that most likely solve the issues.
smart tv = limited cache/memory and no possibility to use vpn.
mag box is old fashion... better get an android box and you will enjoy best flexibility, you can install all apps and also vpn.
e.g. amazon fire tv stick OR xiaomi mi box.
Sounds like TV cache/memory limitation, reduce bouquet list, 2-3 groups only, remove power cable from tv (clear cache) and then try again.
If you need help, feel free to PM me.
No typo in server url, username, password?
Sometimes username or password include small L or capital (big) i letter, have special attention to O and 0 (zero). Try also to reduce bouquet list, in some cases the device has limited cache/memory.
Most smart TVs are very limited e.g. cache and memory.
Better get an Android box which enable you to install all apps, might be also helpful to install VPN :)
most smart tv's having limited cache/memory, and some ISP put speed limits (throttling) on iptv servers... DNS change and a VPN would be helpful in this case, best would be to get an android box where you can install apk files and vpn etc.