Hello,
All new here are about to buy new hardware: I want a secure network, with a backup from a 4/5g router. So it the network is offline, the 4/5g will take over. Is this possible?
Best regards new user 
Pretty sure this would be on your network’s end, not home assistant.
You’d want to get a router with cellular fail over or something similar.
I’ve seen one’s like this and I’m sure there are others out there.
There are a number of 4G routers out there - search for “mobile broadband” or “LTE router”.
With the ones I’ve used the switchover to 4G isn’t seamless - there’s a break of a miniute or so with no signal. Mostly fine for HA but not for streaming, gaming etc. The speeds you get will depend largely on the quality of the mobile signal - don’t blow a lot of cash on it, an expensive router won’t make very much difference.
You need a data SIM card that isn’t capped (a lot of the ones sold for mobile phones have limits on data speeds). The good news is that you probably won’t need a contract - a number of companies provide them on a rolling month-by-month basis.
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Howdy -
I use the TP-Link ER-7206 and the Netgear LM-1200 for exactly this purpose!
I tell the Netgear router that my LM-1200 is a failover connection, only to be used when the primary (my fiber) goes down. The LM-1200 has a Verizon SIM installed, and if I knock down the fiber connection, it does fail over to the Netgear using LTE with no problems.
Let me know if you need any other information!
Sean
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Be aware that switching to a fail over ISP can have some indirect effects also, like certificates on HA failing until the DynDNS have caught up with the changes. Same for portforwarding, which might need extra setup for the fail over line.
If the fail over line use CGNAT and your normal line does not, then you might need to look into services that can work on CGNAT, like a cloudflare tunnel instead of portforward and VPN services with servers on the internet, like Tailscale, rather than having the server locally.
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I use Draytek Vigor2927LTE with build in LTE modem. So if primary line (cable) goes down LTE connection kics on automatically. Works quite well, with ~up to 5sec disruption. Drawback is that indeed my cellural provide uses CGNAT, so it is not possible to use any dynamic DNS service to redirect traffic to new public IP. For connectivity to local network I use Tailscale (though installed on my NAS rather than HA addon). and it allows perfectly to access all devices at LAN with their local IPs. HA works perfectly fine over such connection. If needed to be accessed remotely it also works fine over Tailscale client installed on phone.
The only thing that does not work during such disruption for me is my mail server, as it require public DNS to work. Perhaps using Tailscale MagicDNS in combination of MX records would soleve this, but usually disruptions are not long enought to make it problem.