Linksys WRT54G is likely using WEP or WPA causing "not secure" error.
Check to see if has WPA2 and switch to that in the mean time. (Many old routers have this as part of upgrading the firmware.)
Upgrade the firmware often need Exact Version of X model. WRT54G has many versions. look at the label under them.

quote Originally posted by CoolBlue87GT: No, to both. Modem is SURFboard SB6121

Isn't the best and will slow down the internet.
4x4 modem gets "Download speeds up to 172 Mbps" per https://arris.secure.force....a0ha000000GNcscAAD&c

4x4 = DxU = # of download and upload channels. You want 8x4 or better to match the ISP data speeds.

So you need to upgrade the modem and the router.

AC____ only apples to Devices w/ 5Ghz AC. AC only works on 5GHz and have much shorter range vs 2.4GHz. Worse if has to go thru walls etc.
Otherwise best connection is slower WiFi G or R on 2.4 or 5 GHz bands depending on devices used.

Newer versions of routers/gateways say "better" then AC often only is "better" when have devices support new WiFi formats.

⚠️ Warnings:
● All Gateways including Private Own units are under ISP Control because of the Modem build in to them.
Because All Modems get "Firmware" and other control data by the ISP no matter what ISP "tech support" says.
● Netgear and others now Charge you for support. So Owning a modem etc may not save you money. Worse They only give X days to support just the Router/Gateway then charge for that too.
● Many uses "the cloud" as part of the firewall and other features that's a problem to many too. Like Many Cisco/Linksys had problems because required to login to the cloud to even setup a new unit.
● Yes, Comcast, and likely other, Gateways have Public HotSpots Turn On By Default. To turn that Off, you have to login to Comcast Web, My Account, Devices, to turn off there. You Can't control it in the Gateway interface.

Note:
If you have old iffy cable to home or in a building you will have problems w/ better modems.
DOCSIS 3 and 3.1 modems have much more rules to follow or will slow down or stop working at random times or at all.
Many old buildings have "RG59" cable, crap joints, iffy splitters or too many of them, etc. and all cause problems to the modems.
Your current modem likely have these problems but can "get away" w/ them because it a 4x4 and slow router hiding these problems.
"Login" to the modem at 192.168.100.1 then read https://pickmymodem.com/sig...-3-03-1-cable-modem/
(Nearly all modems including builtin for Gateways uses this address.)
Any problems in a building fix by Comcast will get charge hourly rates by Comcast.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

[This message has been edited by kslish (edited 06-12-2021).]