To upgrade your kernel to 4.19.x on Debian 8 you can use the Rcs Debian Repo. This repo contains a regularly updated version of the 4.19.x kernel. To install this copy and paste the following block of code into your PuTTY or SSH client window when logged into the server and hit enter. This will automatically install the required packages, the gpg key, the repo, the kernel and headers, then remove the old kernel for you. Afterwards reboot the server and you will be on the new kernel.
echo '
#!/bin/bash
# Add GPG Key
wget -O - -q https://debian.choopalabs.com/debian.choopalabs.com.gpg.key | apt-key add -
# Add Rcs Kernel Repo
echo deb http://debian.choopalabs.com/repo/ jessie main >> /etc/apt/sources.list
# Install Rcs 4.19 Kernel
apt-get install -t jessie linux-image
apt-get install -t jessie linux-headers
# Remove old kernel
apt-get autoremove -y
rm -f /root/install-kernel.sh
' > /root/install-kernel.sh
chmod +x /root/install-kernel.sh
/root/install-kernel.sh