The price sounds very good for fibre, was there a fibre connection point outside you home already?
Dell’s are usually good for repairs, and parts readily available, but yes, a steady hand is required 
You can use solder glue to make the join (https://www.amazon.co.uk/Conductive-Wire-Glue-Paint-Soldering/dp/B00SRIITQ4) and have used this in the past but more for electrical breaks than wifi circuits etc.
With your ASUS RT-AX88U, best to disable AirTime Fairness as this maybe what is causing the speed issues via wifi when a few wifi clients are connected.
If you are using Smart Connect, you could be jumping between wifi bands whilst moving around etc.
Also AiProtection can cause the speeds to decrease /increase CPU usage etc. so be aware if using this option for content filtering etc.
In a completely sane world, madness is the only freedom (J.G.Ballard).
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