It appears we have a conflict in infomation.
M02@rt37 and @Georg Pauwen declare it's not possible, yet @Nasir says it is possible, and provides an 881 configuration example.
It's been a while since I've used an 800 series ISR, but I recall (?) for an 881, one of it's FE interfaces (4) is the "WAN" interface, and its other four FE interfaces (0..3) are "LAN" interfaces.
The "WAN" interface is, more-or-less, a typical router interface, and might support subinterfaces as shown.
The "LAN" interfaces are L2 switchport interfaces and do not support subinterfaces. They might support trunk port configuration, but I'm unsure.
The 800 was intended as a combo of a small L2 switch and small ISR for very small branches. I.e. the "LAN" interfaces would support local clients, while the "WAN" port supported the WAN connection.
So, normally, a configuration like shown by @Nasir wouldn't be used.
Also keep in mind, the 800 series has low L3 forwarding throughput capability. For its L2 ports, they might have supported all those ports at wire speed.
From an old Cisco datasheet:
And from a later Cisco White Paper, recommending WAN bandwidth limits:
From the above, although even the "WAN" interface was a FE, even the "fastest" of the 800 series, likely couldn't support, at wire speed, 100 Mbps, duplex.
So, the lack of L3 performance would be a major reason why multi VLAN routing, such as using subinterfaces, would be unlikely configured on such a "router".
