New n95/ n100 4-port (up to 2.5GBASE-T) mini PCs range from 130-250 EUR (when shipped directly from .cn, local distributors tend to be more expensive). Add OpenWrt supported wireless routers/ APs as needed (starting around 15 EUR for e.g. dap-x1860, covr-x1860, wsm20).
Yes, this does exceed your 100 EUR budget, but 1 GBit/s WAN speed isn't easy, even less with VPN, docker, adguard in mind (I would advise against overloading your router with server tasks (docker) for security reasons).
May offer cheaper (used) alternatives, especially towards the later parts of the thread (the pcengines apu devices with AMD jaguar cores are not sufficient for your requirements - and with VPN (and especially docker, just don't...) you really should look at n95/ n100 rather than at 5-10 year old Atom SOCs (and even less AMD jaguar)).