I didn't use Fulcrum with a block explorer so I can't confirm that. But it should work.
After giving up WSL (including electrs and BTC RPC Explorer) I no longer reinstalled a block explorer. I am no longer that much active and I also had much less transactions (hence less to check). So what I'll tell are just theories:

* there's a good chance may not be able to connect as long as Bitcoin Core and Fulcrum are not fully synced; I'd expect you get data only after you see the lines I wrote about in the topic start post

* tcp://localhost:50001 looks good to me, I also use 127.0.0.1:50001:t for my electrum when I connect to Fulcrum


* I did use BTC RPC Explorer with Electrs (see this) for quite a lot of time, but sadly last time I've tried something from my steps no longer worked and I gave up. But I think that since then the Electrs doc is more complete and I've avoided Docker for personal reasons and that may be the easy way.

* I recommend Bitcoin core's blocks\index, chainstate, indexes\txindex and Fulcrum's data folder to be on SSD, else it'll be slow

I've used Fulcrum but can't remember why I it didn't work so I gave up. It could've been down to my SSD or OS config so you'll have to troubleshoot until it's working. Did you locate alternatives ?

TL;DR
Local instances of BTC RPC Explorer and mempool.space work nicely with Fulcrum.

I'll have to dig out how it's configured as I've used it with RaspiBlitz where you simply install it (ok, switching from electrs to Fulcrum needs a tweak or two in earlier versions of RaspiBlitz).

I've used Fulcrum in my RaspiBlitz node setup instead of electrs and had local instances of blockchain explorers like BTC RPC Explorer aka bitcoinexplorer.org and mempool.space. It has worked like a charm until I ran out of space on the 1TB storage media (Fulcrum takes considerably more space than electrs).

From my own experience electrs is significantly slower for large address histories than Fulcrum and this was for me the main driver to switch from electrs to Fulcrum.

Why am I talking of past stuff? Well, I've been a bit lazy to upgrade my RaspiBlitz node and setup a more recent version of it. I also wanted to see how RaspiBlitz evolves (I started with v1.7.0 upgraded a few versions from there and tweaked it here and there to my liking... until it broke). As I've another node with electrs as Electrum server and BTC RPC Explorer and local instance of mempool.space there wasn't much pressure to fix my RaspiBlitz as soon as possible, so I procrastinated...

As far as I can tell from syncing my Fulcrum: it took a while and syncing it is more time consuming than with electrs. But my Bitcoin Core was in sync back then. Fulcrum definitely won't respond to queries as long as it isn't fully synced and as long as the Bitcoin Core it talks to isn't synced either.



The issue might not be your internet connection but additionally maybe too small value for dbcache due to not enough free RAM available. On what kind of storage media is your blocks and chainstate directories located, hopefully a SSD?