I am not clear what you mean when you talk about Softether VPN Client. Can you provide clarification?
I have built Softether VPN server in Raspberry Pi.
Now I want to make one of my Cisco routers as VPN client. Basically, I want IL2TPv3/IPsec VPN client in the router.
This link gives you idea about softether:
A scenario is defined in file attached.
to verify support of L2TPv3 on IR829 we need to look at the feature navigator
it can be used in different ways you can make research by features (top left option) it will open a new browser window
define a filter containing the string L2TPv3 then you can choice "L2TPv3 protocol " feature and search under the IOS section for example for release 15.6.1T2 you can see that the supporting platforms include C881, C886, C887, C891 C892.
So I am afraid IR 829 does not support L2TPv3.
Be also aware that the configuration example in the link that you have reported contains some errors for the Cisco part.
the Cisco IR829 datasheet mentions L2TP but not L2TPv3
Hope to help