Getting disconnected from VPN quite frequently, on AR750 Creta

I have an AR750 Creta. OpenWRT only. I set up VPN, with a bunch of VPN connections, but only one active. I have NordVPN as well as PureVPN.

The VPN connections work, but I quite often get disconnected from VPN. I can still ping the router, but not further. I then disconnect the one VPN connection in Luci, and start it again, then almost immediately the ping goes through again - for some time, when I get disconnected again.
This appears to happen on both NordVPN as well as PureVPN.

Does anybody know why?
Could it be the Creta has not enough power for VPN?

Is there any log when vpn disconnect?

It disconnect while the router is working normally or after reboot?

I currently have no log because I cleaned the AR750 and installed new firmware.
But it disconnected whilst running normally, uptime between days, sometimes weeks, sometimes hours when I am new at a hotel