Horrible internet speed

Since I had a power surge 2 weeks ago I haven't been able to reach anywhere near the 35mbps I purchased through Time Warner (Now Spectrum)

I thought it was the modem they provided was somehow affected so I bought my own compatible modem.
An Arris Surfboard SBG6580 with wifi capability.

My speeds are on average 2mbps and for only a moment it reached 21mbps while the time warner agent "Jump started the connection."

Should I use my Linksys router? Are there any settings I should tweak to get a decent speed again?

They are sending out a technician in a few days if the problem persists to check the lines outside which the power company took down 3 times to replace all the poles on my street.

I would say there is no point doing anything until the lines have been checked , wait until they've cleared any issues outside.

Otherwise you could spend hours looking for solutions only for a faulty connection / wire externally to be replaced and it all work fine.

If they check the lines outside and all is fine, then you can look at possible issues internally...