Help with router/firewall settings, securing my server.

A little history... Since November I have been attacked three times with ransomware forcing me to restore my entire server from bare metal backup's I had off site. The last attack was a little different which just happened on Wednesday(18.02.28). This time they deleted a ton of files and put whatever they didn't delete into .RAR archives password protected. This is getting out of hand now and I need some help securing my server. I did have my static IP address changed the day after the last attack, so maybe that'll buy me some time to get the security figured out on my server.

Windows 2008 R2, Netgear Nighthawk R7000 router.
I also have Malewarebytes Anti-Exploit, Malewarebytes Anti-Maleware and Kaspersky Small Office Security % Installed.

I need to have the ability to log into my server remotely using RDP but I want to make sure my IP, or whichever IP's I list are the only ones with access.

IDK who designed the software for this router but from what I see there's not much in the way of firewall settings. I haven't even seen anything that actually says firewall. After doing some reading it seems like I do have the ability to flash different firmware to my R7000 router like [Tomato, DD-WRT (Kongs) and AsusWRT-Merlin] but since I know nothing about those I don't know if any of them would give me what I'm looking for.

I don't know how much something like this should cost but if it's something you can help me with, let me know what you'd need.

Mike