If you simply and predictably need it for CLIENT MODE ONLY, then I think Dump AP mode is probably the required config. You ideally will want to procure an accesspoints and if you want to be "future proof", then wifi 6 (ax) it is.
As an alternative, you can try to go for a router and use it as accesspoint. It depends on the country you are living in, but in Germany (EU), finding routers with wifi 6 (802.11ax) under 30 € is basically impossible right now (see here). Finally found some < 30€ wifi 5 routers (See here)
If you go for routers/accesspoints, you will either have to go for the cheapest there is, or fish for cheap used stuff on e-bay or the like and not all of these devices are supported by OpenWrt.
Another alternative would be to directly buy a network card and place it in your computer/laptop, without connecting that card via cable to the modem and instead connect to your university wifi directly. See here for a list of wifi 6 capable network cards. Not all of them may be properly supported in the Linux kernel. Same thing you can do here is to buy a usb wifi device (they are cheap) and plug it into your laptop or whatever. See here for usb devices. They are cheap too and you will find some good devices with mt7921 chipset.
To be honest, I don't really understand your network. Is it correct that the modem provides the university wifi? Is the modem really needed in your setup? How many devices should connect to the modem? What Operating system are the clients using?
Is following correct?
university internet --> modem --> router --> client devices
Or is following true?
university wifi --> client devices
Okay I explained myself wrong (I am not good with English). I want to connect my pc with ethernet, but the university router has no ethernet ports available. I was thinking of buying a router that connects to wifi so that I can connect ethernet to it.
No worries, this is a great community, we're all here to help. Welcome to OpenWrt, btw.
Then yes, the client bridge link above is what you need.
Client bridge is very good, and almost everything works, but it's not perfect. There are some troubles with services like AirPrint. So if you have a printer connected to the university wifi, and an apple computer connected to the ethernet, then there may be glitches.
I've used it several times. If you have any issues getting it configured, or questions about the instructions, just ask. If not, feel free to mark this thread solved when you get it working.
I have couple of Netgear EX6150 v2 that I use all the time for access points and clients. They are Atheros based, quad core, can be had very inexpensively and are pretty powerful for what they do. Apparently cheap and easy to get in Germany:
I have never used Atheros devices myself. Mostly MediaTek. All the MediaTek devices I have used have worked very well for client bridges.
This unit is MediatTek, which is good, but it only has 8MB of flash space. Support for devices with 8MB of flash will be going away soon. I'm not sure there is enough room on it to add the packages you would need.
Try and find something with a little more storage space.
Perhaps yours uses a later version of the Netgear factory firmware that requires a signed firmware. See if you can work out what version it is. Try installing an older one from here. (Italian version. )
If that works, see if that earlier version will allow you to install openwrt.
Good luck, hope I didn't steer you wrong with my recommendation, but these really are working well for me.
You can check to see if it's supported by OpenWrt.
If it's not, you can also try using it as a wireless client (aka bridge) in "extender" mode using the OEM firmware and the little switch on the side, if you don't mind that it's not open source. I've never tried using one this way but it's supposed to be a supported use.
(But the seller owes you your money back.)
(By the way, whenever I buy a used device on ebay and there are plenty of choices, I try to pick one where you can see the product information plate or sticker in one of the pictures, because sellers do get this wrong sometimes. I occasionally spend a few dollars more just for this reassurance. I should probably remember to recommend this when suggesting people buy used devices.)