i think, i got this solved https://community.tp-link.com/en/home/forum/topic/259906 because one of the repeaters (a cheap one), spreads the wifi directly from the router to RE450 and it simply won't transmit the ipv6 address. however i understood what DS-lite means (dual stack lite). it's funny that my ISP (internet provider) only give me a ipv6-address but no ipv4-address.

i can ask for one static ipv4-address but i don't want to and ipv4 has no future. china should develop repeaters, where the transmission of ipv6 address shouldn't be a problem. fun fact is that the cheap repeater only spreads a ipv4-address which is funny because i don't have one (it's generated from the ipv6-address and i can't set the ipv6 address transmission on the cheap wifi repeater, i can't open the web surface with the internal ip-address).

now a big problem appeared i think i know the reason. i updated my RE450 V2.1 (DE) to the latest firmware version (i'm sorry that i did it with wifi, but i think it worked) and i told the repeater to access the router directly with 5ghz network but the 2.4 ghz has to be used for the cheap repeater.

this way i have a fast and direct connection to the first router on with 5 ghz wifi on the one hand, and on the other hand i can access a SECOND router with the 2.4 ghz wifi. the second router is quite interesting, because other devices like security cams are connected with the wifi of the second router and there is a pendrive connected to the second router and the security cams record their video 24 hours a day on the pendrive. i am very satisfied with this solution. the two routers are the same models. the first one has the ip 192.168.178.1 and the second one uses the wifi of the first router. the second router somehow DETECTS the first router and changes its ip-adress to 192.168.188.1. so the second router has the ip-address 192.168.188.1. both routers spread internal ip-addresses to every device with DHCP.

since i updated my RE450 (DE) V2.1, the connection to the 2.4 ghz gets disconnected. i can use the 2.4 ghz wifi afterwards, but when i want to access the router, it uses the 5 ghz wifi and i can only access the first router.  

i don't understand why it looses connection to the second router with 2.4 ghz and keeps the connection to the first router with 5 ghz wifi and not vice versa.

network looks like this

internet

- first router (fritzbx 7490)

- - cheap wifi (2.4 ghz only)

- - - second router (fritzbox 7490, many devices are connected to this router because i used the second router as my first router a lot. now i have fiber in the basement and that's why i needed a first router)

- - - - mesh repeater (fritzbox 7430)

- - - - - tp-link RE450 2.4 ghz

- - RE190 (i found a good price for this router, bough it, deactivated it DHCP and now i can access my first router on 5 ghz too)

- - - RE450 (5 ghz wifi connected with RE190 and the 2.4 ghz wifi should connect with fritzbox 7430, sadly it disconnects after a few hours, probably of new firmware and the lost ability to work with two different routers and two different ip address ranges)

new firmware also made the wifi-speed slower!