If the printer picks up other networks—in your case, not just the network settings, everything—the first thing I would do is do a full factory settings reset. If you have anything custom set, take notes of the changes, then rebuild. Some printers allow you to backup and restore, but that could cause the problem to come back in your case. If that doesn't work, see if it works over Ethernet as a test to make sure the networking is at least functional that way.
If the wireless is still not being detected after that and you've already checked the router connection and found nothing at fault, there's nothing in the Brother SM related to the wireless card that I can see that isn't risky or damaging to other critical data like the meter, S/N, and WIC count :-(. I tried—I found nothing. The only other option you could look into is resetting your router to factory settings, or if you have reserved IP addresses, dropping the reserved IP for your printer, letting it get a new one, and then re-reserving it. If it keeps having issues, the card may have been running on a silent failure for years that the power outage exposed when the power was cut and it lost the connection. It looks like Brother hasn't integrated anything in the maintenance mode to test the WLAN card or forcefully reset it back to factory when things like this occur.
Anyway here's the SM: Brother Dcp J4120dw Mfc J4320dw J4420dw J4520dw J4620dw J4625dw Service Manual
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
1
crwdns2944067:05crwdne2944067:0
@serenarihari let's see if our printer guru @nick has a solution for you.
crwdns2934271:0crwdnd2934271:0 oldturkey03 crwdne2934271:0
@oldturkey03 Lemme see if the SM is accessible for this one, given that Brother is potentially conceding and making it easier to find them or isn't DMCAing them when others host the SM. I am investigating to see if there's a reset in the maintenance mode that doesn't wipe out the machine's meter and serial number.
The last time I had a Brother was years ago, when someone handed me one that needed a major maintenance kit. I couldn't afford it, as it was one of the early color lasers, and everything down to the ITB was a wear item. Yes, even the fuser.
crwdns2934271:0crwdnd2934271:0 Nick crwdne2934271:0
@nick SM added to the device page Brother MFC-J4420DW under Documents header
crwdns2934271:0crwdnd2934271:0 oldturkey03 crwdne2934271:0
@oldturkey03 Yeah, there's nothing in that SM for the WLAN :-(. Brother doesn't have a test mode that isn't destructive to other settings the OP may not be able to restore. We were looking at the same SM.
Seriously, there's no test mode for the WLAN card. The reason I'm reluctant to say, "Yeah, wipe the settings out," is that Brothers DO give some warning about the waste ink level, and if the OP wipes it, it'll just up and die when the sensor trips.
crwdns2934271:0crwdnd2934271:0 Nick crwdne2934271:0
@oldturkey03 The good thing is this has Ethernet so if the wireless is shot there’s a way out. It may mean relocation or a long cable. Still better than the bin.
crwdns2934271:0crwdnd2934271:0 Nick crwdne2934271:0