Sharing PC's dial up (PPPOE) connection to a wifi router

Are you on Windows 10? You can Google "Windows 10 Internet Connection Sharing" and try some guides. I've done that with earlier versions of Windows (7, specifically), which were reasonably well set up to do exactly what you're talking about. From the reading I've done, Windows 10 doesn't make it so easy. If you're really committed to this solution, you're going to have to hope that someone comes along who's done this more recently (I haven't), or do a bunch of reading and research yourself. Key things to think about are: which device is providing DHCP and DNS (you need these, but only one device should run these services). At the moment your wireless router will be running these, but you'd need to switch them off. You'll also want to put all your IP addresses on the same network and *may* need to bridge the NICs on your PC - it depends how Windows 10 implements connection sharing.

What I'm trying to say is: while there's a good chance some people have got this working, I don't believe it is straightforward and is certainly not the normal, or best way to achieve what you're hoping to achieve. I know you didn't want to hear @ss202sl's answer (buy a better router), but it is actually the right suggestion.

TP-Link offer a few wireless routers at good prices that get very close to gigabit internet connections. You could replace your D-Link unit with that and plug your PC in. All the ports are gigabit and the router can provide very close to (but not quite full) gigabit internet to wired clients, like your desktop.
Here's an example for the ~$80 mark: https://www.newegg.com/Product/Product.aspx?Item=N82E16833704177&cm_re=TP-Link_AC1750-_-33-704-177-_-Product

Another option is that you keep your current wifi but disable DHCP & DNS, and get a wired router. Ubiquiti offer the very cheap EdgeRouter ER-X for $60. You'll need to enable Hardware NAT in the settings (it's off by default, I don't know why), but once that's done it'll manage gigabit internet for you. You can then connect your PC and get full speed internet and connect your wifi via cable, to provide wifi clients with up to 100mbps.
https://www.newegg.com/Product/Product.aspx?Item=0XK-000W-00080&cm_re=ubiquiti_edgerouter-_-0XK-000W-00080-_-Product
It's not a super-straightforward option either, as the Ubiquiti software takes a little bit of getting used to. But it'll be easier than trying to this through a PC and also gives you the "right" tool for the job (i.e., you're getting something that is purpose designed to provide the service you want).