(How) is it possible to sign in with username/password automatically to a free wifi hotspot?
I live in germany and we have a free wifi hotspot from hotsplots in the neighbourhood.
With a windows client every time I shutdown the client I have to call the hotsplot login-page with a browser to login. Is there some kind of workaround for my smart router to avoid this procedure?
Thanks a lot for helping.
Avoiding isn't possible (mostly) and it highly depends on the captive portal that is used. So there is no general solution for that. However the GL device will try to assist you, read this Connect to a Hotspot with a Captive Portal - GL.iNet Router Docs 4
There might be a openwrt based package called travelmate which may can automate the input of credentials on the free wifi page.
Usually its fine without travelmate, since their router does not know about the clients of your router they only see the dhcp ip from your router.
So to them it acts as one device, so if someone on your own network puts it in, it should work to other devices aswell in your own network, but if the cooldown is very short (the cooldown to be active) then you may want to look into something like travel mate.
Though i have no experience with their package i only know it exists. 
Maybe you can also use the newer captive portal feature inside the repeater settings for captive portals this might just keep the cooldown session active by forging a http request (i believe it does that), then you only have to put it in once.
Like @xize11 mentioned, there is a package called travelmate. This is nothing that works out of the box. You have to write a config for each captive portal. Once you got it working it is rock solid. There are several examples in the package to learn from.
I'm using it in the RRX Trains in Germany where you are limited to 200MB/day. I have a script to change the MAC every 200MB and then travelmate takes care of the captive porttal to relogin.
Can I use this addon with the original firmware ore do I have to change to clean openwrt?
No need for clean openwrt firmware. Glinet firmware will do. Log into luci and install.
Thanks a lot. Which travelmate packages do I exactly need to install in LUCI?
Sorry for the late reply. Been travelling.....
You need:
There is a chinese translation just in case: luci-i18n-travelmate-zh-cn
RTFM: [OpenWrt Wiki] Travelmate, a connection manager for travel routers
Check the included examples they are very helpful.