Swapping cables:
Very likely that you have a number of network devices connected by Ethernet cables. Modem ---Ethernet cable ---> Router ------Ethernet cables to -- ---- > wired devices such as computers, printers, switches, online storage etc....
So what you need to do is to replace each cable along the path with a known working (at speed) Ethernet cable. Between Modem and Router for example. If the speed returns to normal then the original cable may be bad. If speed does not return, then put the original cable back and again use the known working cable to replace the next Ethernet cable in the line - between router and your computer.
Objective is to methodically work through all of the cables to determine if one is bad. It could be true that multiple cables are bad but unlikely unless all of them happened to have been purchased from some low quality source.
Look at each plug - it will not take you too long to get a sense that some cables are better made that others by just looking how the wire conductors fit into the plug. How neat, square, and secure the plug is to the wire. There are standards as to how the wires are actually ordered: 568A and 568B.
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Ethernet cables are round and contain 4 pairs of twisted wires. Each pair twisted with its' counter part and all four pairs also twisted. Good quality Ethernet cables meet certain standard for both twisted and composition. The conductor should be pure copper and a certain diameter that being 22-24 AWG (American Wire Gauge).
Cheap cables are not twisted, made of aluminum with just a copper "cladding", and probably thinner wire than the Ethernet specs require.
Wires can be un-shielded, shielded, stranded, or solid.
Flat cable is usually not twisted, likewise thinner conductors, and overall not suitable for use.
You can easily google for more information about Ethernet cables.
For example:
https://www.comms-express.com/blog/ethernet-cables-explained-all-you-need-to-know/
https://www.tripplite.com/products/ethernet-cable-types
Many other links can be found and some are mostly just advertisements. Some simply are hype and promoting cables that are "rated" for specs not even yet approved.
Ethernet cables have their specs continuously printed along the length of the cable. But the counterfeiters and low end manufacturers simply print bogus information....