netplan - Network with DHCP configured, but no internet
Sorry this is 8 months later than you posted - but I've encountered the same issue and was googling to see if others had solved it.
A few months ago I was experimenting with wireguard and set up a private VPN between two houses - all was well and so I modified my DHCP Server configuration to push the static route (to the other house) out via isc-dhcp-server
This works fine except none of the Ubuntu 18.04 and 20.04 VMs will set the default route now.
If you comment out any option rfc3442-classless-static-routes option ms-classless-static-routes from your DHCP Server conf and restart, then netplan apply - you should find the default route comes back.
Repeat with them back in - the default route disappears.
netplan ip leases ens18 - does show it knows the default route - it just never configures it.
RFC3442 says:
Many clients may not implement the Classless Static Routes option. DHCP server administrators should therefore configure their DHCP
servers to send both a Router option and a Classless Static Routes
option, and should specify the default router(s) both in the Router
option and in the Classless Static Routes option.
because previously it says:
If the DHCP server returns both a Classless Static Routes option and a Router option, the DHCP client MUST ignore the Router option.
So DHCP Client is doing the right thing and not setting a default route - because you are using option rfc3442-classless-static-routes
The solution is to prefix the default route with (in your case) 0, 10, 0, 0, 1,
On my network it is:
option rfc3442-classless-static-routes 0, 172, 16, 0, 1, 22, 172, 16, 4, 172, 16, 0, 250;
which means default route is 172.16.0.1 and the 172.16.4/22 network can be reached via 172.16.0.250
On a client after a netplan apply:
# ip route show
default via 172.16.0.1 dev ens18 proto dhcp src 172.16.0.220 metric 100
172.16.0.0/22 dev ens18 proto kernel scope link src 172.16.0.220
172.16.4.0/22 via 172.16.0.250 dev ens18 proto dhcp src 172.16.0.220 metric 100
Hope this helps others who've been searching and might come across this answer.